The gemora says that one can bench for all if they are reclining or if they all say lets sit and eat, but if they don't say that and they start eating at different times and during the time they eat they talk with each other is it as if they are eating together and could they make a mezumon?
hananiah lasker, philadelphia,pa - amsterdam,holland
According to the Shulchan Aruch (OC 193:2), if there was Kevi'us Se'udah by at least one person and others later joined him then they must be Mezamen. In the event that there was no Kevi'us Se'udah at all, Zimun is optional.
The Mishnah Berurah (ibid. 24) quotes the Magen Avraham who questions this ruling and maintains that if there was no Kevi'us Se'udah together they cannot be Mezamen.
Kevi'us Se'udah means that they all prepare themselves to eat at the same table (Mihsnah Berurah ibid. 21).
In all cases one should contact his local Orthodox Rabbi for a ruling, since there are various additional factors which must be considered.
Best wishes,
Y. Landy
Regarding that which you mentioned that in a Yeshiva dining room, even when starting at separate times it is considered a kvius regarding zimun, I just wanted to point out that in the Philadephia Yeshiva, al pi the psak of the Rosh HaYeshiva HaRav Shmuel Kamentsky Shlit"a, when meals are eaten in such a fashion, no zimun is recited as it is not considered a kvius. The only time it is considered a kvius, is when everyone starts together (i.e Shabbos meals).
I understand that this is not the common practice in other yeshivas, but I wanted to add this to the discussion.
Avi Kappel
Brooklyn, NY