Regarding Fulfilling the Obligation of Birkat Hamazon for Others

Greetings, If there are only two people, initially one should not discharge the other from the obligation of Birkat Hamazon unless the other does not know how to bless himself. In such a case, one can intend to discharge him from the obligation, provided the other intends to listen attentively to every word (see Shulchan Aruch Orach Chayim 186:1 and Mishnah Berurah 2, 5). If there are more than two people, in principle, an adult who has eaten to satiety can discharge others from the obligation, provided that the one who recites intends to fulfill their obligation and the listeners intend to fulfill their obligation through him, and the listeners must hear every word clearly from the one reciting. Nonetheless, even in such cases, each individual should bless for himself, as there is a common occurrence that guests become distracted and do not direct their intention to the reciter's words at all (Shulchan Aruch ibid. 187:7 and Mishnah Berurah 27). 

Blessings, Hillel Meirs