Amen in Bentching
Let’s now discuss birkas hamazon (grace after meals, colloquially known as “bentching”). Grace starts with a call to bentch, called a zimmun. As part of the zimmun the diners say, “Baruch she’achalnu mishelo uv’tuvo chayinu” – “Blessed is the One of Whose bounty we have eaten and in Whose goodness we live.” This phrase is then repeated by the leader. The question is: should one respond amen after the leader recites “Baruch she’achalnu?”
The Levush writes that if three or ten people have eaten together, the practice is not for them to answer amen after the leader recites “Baruch she’achalnu” (or “Baruch Elokeinu she’achalnu…,” which is the text recited when ten or more have dined together). The reason for this is so that the diners should not be distracted by an interruption and they will all bentch together. The author of the Pri Megadim writes similarly (Mishb’tzos Zahav 198:1) that the one who leads bentching should not reply amen to the diners and when he recites “Baruch she’achalnu,” they should not reply amen so that they can give their undivided attention to starting the first bracha simultaneously with him. However, one who didn’t eat with them and is not participating in the zimmun should reply amen if he comes in and hears the leader say “Baruch she’achalnu.”
Eliyah Rabbah (198:2) writes that if a person who didn’t eat hears the leader say “nevareich she’achalnu mishelo” (“let us bless the One of Whose bounty we have eaten”) and replies with “Baruch u’m’vorach shmo tamid u’l’olam vaed” (“Blessed is He and may His Name be blessed continuously forever”), then that person need not answer amen to the others because his words are the functional equivalent of the “Baruch she’achalnu” that the diners recite. If he didn’t say “Baruch u’m’vorach shmo tamid u’l’olam vaed,” however, then he must answer amen when the others say “Baruch she’achalnu” in response to the leader, and possibly also when the leader repeats that phrase.
The bottom line according to the Chayei Adam (48:17) is, if you’re a non-diner in a room where the zimmun is being recited and you hear the leader recite “nevareich she’achalnu mishelo,” you should respond with “Baruch u’m’vorach shmo tamid u’l’olam vaed.” If you come in after the leader has already said “nevareich she’achalnu mishelo” and you only hear the other diners respond with “Baruch she’achalnu,” then you should respond with amen. If ten or more have eaten together, the rule is the same except then one would say “Baruch Elokeinu u’m’vorach shmo….” If the leader has already recited “Baruch Elokeinu she’achalnu…,” then the new arrival would say amen. [Siman 85]