1. Two groups that eat in one or two houses that can see one another join to form a zimun. If not [they do not see each other], they do not join. If there is one waiter [servicing] both groups, they join [to form a zimun]. This applies when they entered their meals to begin with with the intention of joining together [to form a zimun]. And there are those who say that if there is a public domain between the two houses, they do not combine in this matter.
2. When some of them ate in the house and some of them ate outside the house, if the one leading the blessing sits on the threshold of the house, he combines the two groups.
3. When the two groups combine, the one leading the blessing must be clearly heard by both groups. [Hagah] And each member of the groups says the after blessing himself. But, if they want to fulfill their obligation of the after blessing with the recital of the leader of the blessing, each one must listen to the entire after blessing [recited by the leader]. Without this, (according to all opinions) they have not fulfilled their obligation [to say the after blessing].