309. Who Forms a Group?

45:18 At large gatherings with many diners, one should choose someone with a powerful voice to lead bentching so that everyone can hear. If it’s not possible for everyone present to hear the leader, the diners should bentch in smaller groups of ten each. (If this will be too noisy and the host will object, they may even break into groups of three – OC 193:1.)

45:19 If two groups ate in the one room, or even in two rooms so long as some diners in one group can see some diners in the other, then they are considered one group for the purposes of making a zimmun. If one person waits on both groups, this also serves to join them into one group for bentching. (The Kitzur Shulchan Aruch says that the diners had to have eaten with the intention of bentching together but Mishnah Brurah 195:6 cites opinions that disagree with this.) Whenever two groups are joined together, everyone must hear the leader at least until the end of the first bracha of bentching.