By Rabbi Dovid Ostroff
In the previous column, we discussed the major criteria needed to make a zimun. The issues mentioned were 1- eating together, 2- type of food, 3 – eating in one place, 4 – who joins the zimun.
Briefly, eating together – at least three people dined together, which required them either to begin their meal together or conclude it together.
Type of food – optimally all three would have needed to consume bread, but custom is if the third person will not wash for bread, suffice if he consumed any type of food or beverage (except water).
Eating in one place – They either dine at the same table, or separate tables for lack of space. Please see the previous sheet for more details, such as eating at separate tables in a Yeshiva or restaurant.
Who joins the zimun?
According to the Rama (Ashkenazim), three males above Bar Mitzvah age can form a zimun. 
According to the Mechaber, one child younger than Bar Mitzvah age can join two adults to form a zimun. We find various opinions to the age of this child, some say above the age of six and others say above the age of nine. 
Women – three women are permitted to form their own zimun  and women may answer men’s zimun, but women may not join men to form a zimun, even if they are related.
Are women who dined with men obligated to answer the zimun?
The Shulchan Aruch writes  that women who dined with men are çééáåú – obligated. Based on this, Rav Moshe Feinstein ztz”l writes  that women who are busy tending to a household’s needs, do not intend dining together with other diners, are not obligated to zimun, but on Shabbos, or when they intend dining with everyone else, they are obligated to answer zimun. It is incumbent to begin zimun with all participants present.
If one of the three is in a rush, may he bentch without waiting for zimun?
Once one is obligated to zimun (by commencing eating or concluding together), one may not bentch without zimun.  Consequently, if one of the three would like to bentch and leave, a person may not do so without zimun. In this case, because they are the majority, the other two diners are not obligated to stop and allow the third to bentch, but it is correct for them to do so.
Where two of the three would like to bentch and the third person is still dining, the third must cease dining and answer zimun to the other two and then continue.
In other words, a single person must stop for the other two but two need not halt for the third, although it is correct to do so.
What does ‘stop eating’ mean?
When two people want to bentch, the third must stop eating for the duration of the zimun.  This means that he answers to the zimun and listens to the mezamein until äæï àú äëì, answers amen and may continue dining.  According to the Mechaber, it suffices to reply to the zimun and then continue.
When dining at a wedding or other function, it is all too common that one wants to leave before everyone else bentches, but since one began eating together with everyone one is obligated to bentch with a zimun and one may not leave before that.
Is there a solution to this problem?
Rav Moshe Feinstein ruled that, when necessary, if when one recites the hamotzi on the bread one’s intention is not to join everyone else, one may bentch without zimun. 
In this particular case it is more complicated, because when a quorum of males dine together they are obligated with Hashem‘s name – ðáøê ìàìå÷éðå ùàëìðå îùìå, in which case they may not break up into groups of three, because Hashem‘s name will be omitted.  Twenty people may break into two groups (although according to some poskim, there is merit in this case when more people do a mitzvah together on account of áøåá òí äãøú îìê – the more people performing a mitzvah together, the more glorious the king). 
So ideally, when some people would like to leave early before the communal bentching, ten people should bentch together and recite Hashem‘s name in zimun. However, since such a zimun would most probably reach the ears of the host and upset him, they may break up into three and bentch the regular zimun, omitting Hashem‘s name.  Yet this should only be done when they have very important issues to attend to or to perform a mitzvah.  If they know beforehand that they want to leave early solely for convenience, they should have it in mind that they are not joining the diners.
Rav Sternbuch shlita added that people should try bentch with a zimun of three even when one had in mind not to join.
 Siman 199:10.
 See M”B 199:24 and ùòøé äáøëä ô”ä äòøä ëå.
 Siman 199:7.
 àâ”î ç”ä è é.
 Siman 193:1.
 Siman 200:1.
 Siman 200:2.
 àâ”î àå”ç à’ ñé’ ðå. Apparently not everyone agrees to this ruling, see åæàú äáøëä ôé”ã äòøä 3.
 Siman 193:1.
 See M”B 193:11.
 Siman 193:1,
 See M”B siman 193:16.