Three people who eat together are obligated to do a "zimun" - to say birkhat hamazon together - and one blesses on behalf of all. In which case would the three not be obligated? What is the source for it? Can two people also do a zimun if they want? Can women do a zimun? Are they obligated to or is it optional? Why can they not join together with slaves to get to three? What happens when three people eat together and one leaves early? Someone who walks into the room while they are saying the zimun, what does he answer? Why do we add the word amen to the end of the third blessing in birkhat hamazon?