In order to say Birchas HaMazon with a Mezuman the people in the Mezuman must eat together. Rav Elyashiv points out two cases on Pesach night where people eating together do not qualify for a Mezuman and must say Birchas HaMazon as individuals without a Mezuman.I don't understand how this is applied, because pretty much anytime I am eating with other people, nobody is going to be eating from each others food. If I am at work, sitting at a lunch table eating my lunch that I brought from home (or even lunch that I bought), nobody else is going to eat from my food, and I am not going to eat from anybody else's food. If I am sitting with other people and we are talking to each other and eating together, should we not be making a zimun when we bentsch? According to the above psak it would seem that we should not.
The first is two people that eat the seder together but one uses only machine matza while the other only eats hand matza. Despite that this is only a Chumra and technically l’halacha each one is permitted to eat the other one’s matza, nevertheless they are not considered eating together for the sake of mezuman.
The second case is people who eat the seder together but each one eats their own food and have a minhag not to eat from anyone else’s food other than their own. Here also says Rav Elyashiv, they cannot make a mezuman together. (Kovetz Tel Talpiyos - Piskei Shmu’os Pesach)
Even at home I will only eat from my kids' plates, but if we have guests I would not eat from their plates. So perhaps even at home with guests we should not be making a zimun.. Maybe if the food is put out in serving dishes and everyone takes from the serving dish to their own plate, then maybe that would qualify for a zimun..
What is especially interesting is that these people, in the shailoh Rav Elyashiv is discussing, are sitting at a Pesach seder together. Not 2 random people sitting in a cafeteria just by chance at the same table. These are friends, relatives, host-guests - they are clearly eating together. yet one has a chumra/minhag the other does not. And still, despite the "kirva" and the definity of them eating together, they are still considered to be eating separately.
And one more point - it is clear from the discussion that just because one person has a chumra the other does not keep is not a reason to avoid eating together. The one guy in the story above could have said I cannot come to your seder because I only eat machine matza and hand matza is chametz to me, or vice versa. You have your chumra, and I have my chumra, but we should try to work it out and eat together...
(HatTip: Matzav.com, via the Wolf)