When only two people eat together, each should say his own Birkas Hamazon. If only one person knows how to bentch, he can motzi the other only under the following conditions:
The listener hears every word. (B'dieved, it’s okay if he only heard the main parts plus the beginning and end of each beracha.)
The listener has kavana to fulfil by listening.
The one bentching has kavana to exempt the listener’s obligation.
There are poskim who don’t agree with the last of the above conditions. They hold that if the bentching (or Kiddush etc.) is in Lashon Hakodesh, the listeners can fulfil their obligation even without understanding it. (This is not true by any other language). This is the minhag. Nevertheless, if they are able to, they should repeat quietly after the mevoraych, the Birkas Hamazon, word by word. (MB5)
All agree that verbalizing bentching in Lashon Hakodesh fulfills the obligation even if not understood. (ShaH3)
Listening to Birkas Hamazon while dozing does not count. (BeH “v’tzorich”)
Two people should look for someone else in the building to eat something with them so they can make a zimun. (ShA1 – MB7)
When three, four or five people eat together they aren’t allowed to split up – they are each obligated in the zimun. (ShA1)
Six, seven, eight and nine people can split up into multiple groups of at least three each. (ShA1)
Ten people until twenty are not allowed to split up because they are obligated to say Elokaynu. Twenty people are allowed to split up into two groups of ten. (ShA1)
It is a mitzva for seven people to get three others to eat something with them so they can bentch with the Shem. (ShA1 – MB12)
At a large seuda there are cases that justify breaking away into zimun-groups of 3 without the Shem: The baal habayis will be upset if big groups of 10 bentch together because of the commotion, but many people cannot hear the Birkas Zimun from the mevoraych. Or else, people need to leave early for a d’var mitzva that cannot wait. (ShA1 – MB16)
How much of the zimun must you be able to hear? At least each word of the Birkas Zimun until “…hazon ess hakol.” (MB17)
At a large seuda, give the honours of leading the zimun to someone with a loud voice so all can hear. (MB17)
If 3 people either began or ended a meal kavua together, they are obligated in zimun. (MB19)
What is considered “kavua – together”? Sitting down, and at one table. A baal habayis with his household members who eat together are considered kavua even without one table. (MB18)
If two people begin a meal together and one person joins later and is still eating after the two are finished –the two may commence bentching without a zimun. However, if the two wait for the third one to finish eating before they bentch, then they are obligated in zimun. (MB19)
If one of the three never sat down to eat, he is not “kavua together” with the other two and they should not do the zimun. (ShA2 – MB24)
Whenever there is a doubt about 10 people eating kavua together, they shouldn’t take a chance on saying Hashem’s name in vain – just do the regular zimun. (MB26)
When saying a zimun in a gentile’s house you can say “Horachaman hu yishlach lonu (not “y’varaych ba’al habayis hazeh”) beracha meruba b’ahalichosaynu uvishivosaynu ad olam”. Alternatively, you can say “…hu y’voraych ess baal habayis hazeh” and it refers to the owner of the seuda. (MB27)
When eating your own food at someone else’s house (even Jewish) say “…hu y’voraych osonu…” (MB27)
Once you answer the zimun but you did not yet bentch, you cannot count for another group of zimun even if you ate more after answering. (ShA5)