Brachos 7:4

If three men ate together, they may not separate to bentch; the same is true for four or five. Six may split into two groups of three; seven through nine may likewise form smaller groups. A group of ten may not separate until there are twenty (so they can make two groups of ten each).

Brachos 7:5

Two groups eating in the same building may join together for bentching if some people from each group could see the other group during the meal; if not, each group bentches on their own. In the time of the Mishna, people used a wine concentrate that could not be drunk until it was diluted. Rabbi Eliezer says that one may not recite the bracha over the wine until it has been diluted; the Sages say that one may.