I don't think the Magen Avraham says that. It looks like 193 discusses a more complex case of people leaving small groups and the remainder reassembling etc.
Shulchan Aruch 195:1 simply says that if groups of people are eating in the same house or even two different houses, if some of the people at each table can see each other then they can combine for zimmun. There is also the idea that a mutual waiter can cause combining. See the whole idea there. But that is the normative, general halachah.
In addition, the Tur says in the name of his father the Rosh, that at a wedding feast, even if the overflow crowd cannot see the groom and is eating in a seperate house, they can join in the blessing of the wedding since they intended to feast along with the groom's party.