- One should say Al Neharot Bavel before benching after a meal and on days when there’s no tachanun one should say Shir HaMaalot BeShuv Hashem instead. The minhag is to say Shir HaMaalot at Seudot mitzvah as well. 
Obligation of Zimmun
- If three people eat together are obligated to make a Zimmun before benching (making Birkat HaMazon).  Chazal based it on the pasuk “גדלו לה" אתי ונרוממה ה" יחדיו”  and “כי שם ה" אקרא הבו גודל לאלוקונו” .
- Most authorities consider Zimmun to be a rabbinic obligation. 
- Zimmun could be said in any language as the purpose is to introduce the benching orally and join the group together to praise Hashem. 
- Some say that answering Zimmun is considered one of a person’s hundred Brachot every day. 
Who has the obligation?
- It only applies when three or more people eat together. The three conditions to be considered “together” is that those who are eating 1) eat while seated, 2) sit at one table, and 3) eat simultaneously either at the beginning or the end of the meal. 
- If two groups ate in different places in the same house and they could see from one group to another, if they have intent (when they began the meal) to join for Zimmun they may join, however, if if they didn’t have intent, then according to Sephardim they may not join for Zimmun, but according to Ashkenazim it’s a dispute whether they may join for Zimmun. 
- If three people aren’t obligated to make Zimmun as they didn’t join their meals, the three can’t do Zimmun. 
- If three ate together and were obligated in Zimmun and one forgot about Zimmun and said Birkat HaMazon then the other two can say Zimmun with the third. However, if one of the group answered Zimmun with another group he can’t answer another zimmun of three. 
- If three ate together and one person left he could be called back and Zimmun could be said even if he’s standing by the door next to them, however he should say Birkat HaMazon where he ate. However, if ten people ate together and one left he should be called back and everyone should sit down for Zimmun. 
- If in a school or yeshiva, the students go to lunch at the same time, the group can make Zimmun together even if they sit at separate tables as long as they can see from one table to another. Even if they don’t have enough at each table to make their own Zimmun of ten, they can join together to make a Zimmun of ten. However, it’s preferable that they one time actual say verbally that they intend to eat together. 
Two who ate together
- Two who ate together aren't obligated in Zimmun and therefore, they should say Birkat HaMazon to themselves. However, if one of them doesn’t know how to say Birkat HaMazon and the other does, then the one who knows may say it aloud and fulfill the obligation of the one who doesn’t know as long as the one saying has intent to fulfill the obligation of the other, and the one listening has intent to fulfill his obligation. 
- According to some it’s crucial that the one who is listening understand the Birkat HaMazon, while others are lenient and say that such is the minhag. 
A Zimmun of Ten
- If ten people ate bread together, they are supposed to add the word Elokenu in the Zimmun.
- According to Sephardim, one doesn't have to stand for the word Elokenu in the Zimmun.  Many Ashkenazim have the custom to stand for the word Elokenu in a zimmun of ten, however those who don't stand have what to rely on.
Two who ate together
- Three who ate together must do Zimmun and each one is not allowed to leave the Zimmun. However, if there’s six or more in the Zimmun, the group is allowed to split into two groups of three. If there’s ten then each one isn’t allowed to leave the Zimmun as they are obligated in ZImmun with a mentioning of Hashem’s name. However, if there’s twenty the group may split into two groups of ten. 
- Some say that it’s always preferable to join together in a larger group while others contend that once one is joining in a group of three or ten there’s no obligation to join in a larger group. 
- If three people ate together in the beginning or they finished together (and it’s considered as if they finished together if they still would eat something had if be brought to them) then there’s an obligation of Zimmun. 
Who can join a Zimmun
- The Ashkenazic minhag is not to include children under Bar mitzvah for zimmun, while the Sephardic minhag is to include children of age 6 who know to whom they are saying Birkat HaMazon. 
Zimmun after someone already Benched
- If three people ate bread together, and one forgot to wait for Zimmun, the other two can do Zimmun with the one who already Benched. 
- If three people ate together, two ate bread and one ate a Kezayit of something else or drank a Reviyat of a drink (other than water) are obligated to make Zimmun. 
- If one of three only had a Kezayit of another food or a reviyat of drinks, and one of those who ate bread forgot to wait for Zimmun and Benched, the other one who ate bread can no longer make Zimmun. 
- If one of three only had a Kezayit of another food or a reviyat of drinks, and the one who didn’t eat bread forgot to wait for Zimmun and made Bracha Achrona, the other two who ate bread can no longer make Zimmun. 
- If there’s a doubt whether Zimmun was made or not (or in general a doubt concerning Zimmun) one should be strict to say Zimmun and in a Zimmun of 10 one shouldn’t add Hashem’s name. 
Saying Birkat HaMazon aloud
- The original establishment was that the one doing Zimmun would read the entire Birkat HaMazon out loud. Nowadays, the practice is that everyone say it silently to themselves. 
- According to Ashkenazim it’s preferable to finish the Bracha before the Mezamen (one doing the Zimmun) and then when the Mezamen finishes answer Amen. According to Sephardim there’s no reason to finish before the Mezamen and if one did, nonetheless, one doesn’t answer Amen.  See further on the Birkat HaMazon page.
- If it’s difficult to listen to the entire Birkat HaMazon out loud one should at least listen to the primary part of the Bracha, which according to Sephardim is Birkat HaZimun (the words “Baruch SheAchalnu…Chayinu”) and according to Ashkenazim is through Birkat HaZan (the first Bracha of Birkat HaMazon). Therefore according to Ashkenazim the Mezamen (even if he is Sephardi) must say the first Bracha out loud and everyone should say it silently along with the Mezamen. 
- According to Ashkenazim, if there’s a large group and those benching will not hear the Mezamen until the end of the first Bracha it’s preferable for the group to separate into groups of three so that it’s possible to hear the Mezamen until the end of the first Bracha. 
- It’s preferable that the Mezamen not use a microphone for Zimmun but rather someone with a loud voice do the Zimmun. 
Procedure and Text of Zimmun
- The Mezamen, who is leading the zimmun, should invite everyone to say Birkat HaMazon. It may be said in any language. If one wants to say this invitation in Hebrew, one should say "Ten Lanu Kos UNivarech" (if one is saying Birkat HaMazon over a cup of wine) or "Bau UNivarech" (if one isn't saying Birkat HaMazon over a cup of wine). If one wants to say this invitation in Aramiac, one should say "Hav Lan UNivarech" or an extended version of this is "Hav Lan VeNivrich LeMalka Iylah Kiddisha", which many Sephardim have the custom of saying. If one wants to say this in Yiddush, the phrase one should say is "Rabbotai Mir Velin Benchin". 
- Those who are participating in the Zimmun should answer to this invitation, according to Ashkenazim, "Yehey Shem Hashem Mivorach MeAtah VeAd Olam", and according to Sephardim, "Shamayim". 
- After the invitation, the mezamen should begin the actual zimmun by saying "Nivarech She'achalnu Meshelo" or with an extended version of "Birshut Malka Iylah Kiddisha UBirshutchem, Nivarach She'achalnu Meshelo", as is the custom of some Sephardim. 
- Those participating in the zimmun should respond "Baruch She'achlnu Meshelo Ubituvo Chayinu." 
- Lastly, the mezamen should conclude "Baruch She'achlnu Meshelo Ubituvo Chayinu."
- If one hears Zimmun but didn’t eat should answer “Baruch UMevurach Shemo Tamid Leolam VaEd.” (ברוך ומבורך שמו תמיד לעולם ועד). 
- Kitzur S"A 42:5, Mishna Brurah 1:10,11, Piskei Teshuvot 1:14 in the footnote. See also Rabbi Aryeh Lebowitz on the Ten Minute Halacha.
- S”A 192:1
- Tehilim 34
- Devarim 30. Gemara Brachot 45 quoted by the Mishna Brurah 192:1.
- Pri Megadim (A”A 197:2) writes that most authorities consider zimmun to be of rabbinic obligation. Chaye Adam 48:1 writes that Zimmun is derabbanan and some say it’s Deoritta. Chazon Ish (31:1) argues that Zimmun should be Deoritta.
- Zohar (Balak pg 186b) writes that it’s important to precede benching with “give us a cup to bench” in Hebrew or Aramaic to introduce the benching to bring the kedusha. Mishna Brurah 192:2 quotes this and writes the minhag ashkenaz was to say Zimmun in Yiddish “Rabbotei Mir Velin Benchin”. Kol Bo (Siman 25) emphasizes the group merit of the zimmun.
- Sefer Keysad Mezamnin 1:22
- S”A 193:2 writes that a third person can join two that already started eating if he is “Koveh” (establishes his place) with them. Mishna Brurah 193:21 writes that if one doesn’t eat while sitting and eat at the same table one isn’t considered as being Koveh with the others. Magan Avraham 195:2 and Mishna Brurah 197:3 also include the requirement of sitting at the same table.
- S”A 195:1, Mishna Brurah 195:6
- Rama 193:3 writes that even if three people aren’t obligated in Zimmun it’s preferable that they make Zimmun because of Berov Am Hadrat Melech, that it’s preferable to honor Hashem in multitudes. Mishna Brurah 193:23 writes that the same would apply if there are more than 3 people. However, Magan Avraham in name of many poskim that since one can’t fulfill the Birkat HaMazon for another person one is also not allowed to make a zimun together. Mishna Brurah 193:24 concludes that the Magan Avraham is more logical.
- S”A 194:1
- Rama 194:1
- S”A 194:2, Mishna Brurah 194:8
- Yalkut Yosef (vol 3 pg 375)
- S”A 193:1
- Mishna Brurah 193:5
- S"A 192:1
- Halacha Brurah 192:12
- Sh"t Bear Moshe 1:2
- S”A 193:1
- Rama 193:2, Mishna Brurah 193:11
- Mishna Brurah 193:19
- Rama 199:10
- S"A 199:10, Sh"t Yechava Daat 4:13
- S”A 194:1 writes that if one out of three people in a Zimmun said Birkat HaMazon they can still ma`ke Zimmun to fulfill the obligation of the two and the one who said Birkat HaMazon will not fulfill his obligation because a Zimmun must be said before Birkat HaMazon.
- Mishna Brurah 197:20 writes that if one out of three people ate a Kezayit of a food (other than bread) or drank a reviyat of drinks (other than water), the three people can still make Zimmun.
- Mishna Brurah 197:9
- Mishna Brurah 194 (Buir Halacha D”H Echad Mehem) quotes tha Eliya Rabba who is unsure whether the one who ate bread and didn’t make Birkat HaMazon can make Zimmun and concludes that since there’s only one who is obligated (and can fulfill their obligation) they can’t make Zimmun. Such is also the opinion of the Birkei Yosef, Mamer Mordechai, and Magan Giborim.
- Mishna Brurah (Buir Halacha 197 D”H Imahem) quotes the Pri Megadim and Chaye Adam who write that if there’s a safek by zimmun one should say it however by the Zimmun of ten people one shouldn’t say it because it includes Hashem’s name.
- Vezot HaBracha (pg 128, chapter 14) quotes that Panim Meirot that the original establishment was that one person say it aloud and everyone listen. However, S”A 183:7 writes that the practice that should be followed nowadays is that one person say it aloud and everyone to follow along Bracha by Bracha.
- S”A and Rama 183:7, Vezot HaBracha (pg 128, chapter 14)
- S”A and Rama 200:1 identify clearly the crucial part of Zimmun, according to S”A it is the Zimmun of Baruch SheAchalnu and according to the Rama it is including the first Bracha. Mishna Brurah 183:28 holds that Ashkenazim should at least say the first Bracha quietly together with the Mezamen and those who have the minhag that everyone just says it to themselves are mistaken. So rules Vezot HaBracha (pg 129, chapter 14). Vezot HaBracha (pg 335) quotes Rav Wosner in Kovetz MeBet Levi (Nissan 5758) who defends the minhag slightly. Yalkut Yosef (vol 3 pg 371) says that a Sephardi who is a Mezamen for Ashkenazim should say the first Bracha out loud.
- Mishna Brurah 193:17, Vezot HaBracha (pg 129, chapter 14) also quotes the Chazon Ish who argues but still quotes the Mishna Brurah as the primary opinion.
- Vezot HaBracha (pg 129, chapter 14)
- The Magen Avraham (Intro to 192) quotes the Zohar which says that words of kedusha require preparation and that is the basis for the minhag to invite everyone to say Birkat Hamazon. The Magen Avraham writes that the Ashkenazic minhag was to say it in Yiddish with the words "Rabbotai Mir Velin Benchin". The Mishna Brurah 192:2 adds that it may be said in Hebrew with either the phrase "Ten Lanu Kos UNivarech" (if one is saying Birkat HaMazon over a cup of wine) or "Bau UNivarech" (if one isn't saying Birkat HaMazon over a cup of wine). The Kitzur S"A 45:6 adds that this invitation could be said in Aramiac with the words "Hav Lan UNivarech." Rav Mordechai Eliyahu (comments to Kitzur S"A 45:12) writes that the Sephardic minhag is to say this invitation in Aramiac with the words "Hav Lan VeNivrich LeMalka Iylah Kiddisha."
- The Magen Avraham (Intro to 192), Mishna Brurah 192:2, and Kitzur S"A 45:6 records the Ashkenazic minhag to answer the invitation with the pasuk "Yehey Shem Hashem Mivorach MeAtah VeAd Olam". Rav Mordechai Eliyahu (comments to Kitzur S"A 45:12) writes that the Sephardic minhag is that those participating in the Birkat HaMazon answer "Shamayim," meaning, that this should be done with the permission of heaven.
- S"A 192:1 writes that the mezamen should begin the zimmun by saying "Nivarech She'achalnu Meshelo". Rav Mordechai Eliyahu (comments to Kitzur S"A 45:12) writes that the Sephardic minhag is the mezamen starts the zimmun with the words "Birshut Malka Iylah Kiddisha UBirshutchem, Nivarach She'achalnu Meshelo."
- S"A 192:1 writes that those participating in the zimmun should respond "Baruch She'achlnu Meshelo Ubituvo Chayinu."
- S"A 192:1 writes that the mezamen should conclude "Baruch She'achlnu Meshelo Ubituvo Chayinu."
- S”A 198:1, Vezot HaBracha (pg 129, chapter 14)