504. Breaking Up a Group

Brachos 5:10

When three people eat together, they may not divide themselves and break up the zimmun. The same is true of groups of four and five. Groups of six through nine may split up into two separate, smaller groups. When there are ten or more, they may not separate until there are twenty (so that they can form two groups of ten). The general rule is that any smaller groups that form must be able to recite the same zimmun as if the large group had stayed together.

Brachos 5:11

If three people came from different groups of three and joined together, they may not separate unless each of them already participated in a zimmun with his previous group. One is no longer obligated in zimmun once he has already participated. When three people eat bread together, they may not separate even though each of them ate his own food.