507. Bentching for Other People

Brachos 5:16

A man can only fulfill his obligation in bentching by listening to a woman, a servant or a minor when he has not eaten to the point that he is sated. In this case, his obligation is only rabbinic in nature and can therefore be fulfilled in this way. If he ate until he was satisfied, he is obligated to bentch by Torah law and he cannot fulfill his obligation by listening to a woman, a minor or a servant. Anyone who is obligated in a mitzvah by Torah law can only have his obligation discharged by another person who also obligated by Torah law.

Brachos 5:17

Let’s say that a person enters a room and finds others reciting the zimmun. If he entered as the leader says, “Let us recite grace,” the newcomer should respond, "Blessed is He and may He be blessed." If he entered as the other diners were responding, he should simply reply amen after.