506. Bentching Alone

Brachos 5:14

If three people are together and one bentched by himself before the others, the zimmun can still be recited with him. The other two diners fulfill their obligation in zimmun but he does not because the zimmun (the call to bentch) doesn’t work after one has already bentched.

Brachos 5:15

When two people eat together, each one should bentch by himself. If only one of them knows how to bentch, he should do so out loud. The other person should reply amen after each bracha in order to fulfill his obligation. A son may bentch for his father, a servant may bentch for his master, and a woman may bentch for her husband, each of these enabling a person to fulfill his obligation. However, the Sages have said that it is shameful if a man needs his wife or children to bentch for him.