305. Adding a Third to Form a Zimmun

45:10 If two ate together, it’s a mitzvah to ask a third to join them to form a zimmun, even if the third came after the two had already finished eating. However, if they were served another course as dessert, it is a mitzvah for them to include the third person in this course, which would then obligate the three of them to form a zimmun. Some authorities require that the third person eat bread, while others say that any grain product is sufficient. Still others say that even fruits or vegetables are sufficient, while the most lenient opinions do not even require the third person to eat at all. According to these authorities, if the third person drank a reviis (approximately 3.5 ounces) of any beverage other than water, he could join the zimmun. This is the accepted practice. After the leader has concluded the first bracha of bentching, the third person should say the appropriate concluding blessing for whatever it was he ate or drank. If the third person arrived after the two diners have washed mayim achronim, he can no longer be added to form a zimmun. 45:11 If three ate together, because they are obligated to form a zimmun, they are not permitted to disband before bentching. This is also the case with four or five diners; none of them is permitted to bentch on his own since each has an obligation for a zimmun. Six to nine diners may break into two or three groups for a zimmun, but not ten (since they form a minyan). (There is an opinion that a zimmun of six is preferable to two zimmuns of three each because the larger gathering is greater praise to God; others feel that these groups are equal – Mishnah Brurah 193:11.)