Can someone be yotzei a beracha acharona from someone else? Can one be motzi others in beracha acharona? How is it that is done in many shuls across the globe today?
For foods other than bread and wine, it is preferable, even when seated together, for each to make his own blessing. However, when seated together, one person can be motzi the others in any berachah acharonah. Even when not seated together, one it yotzei bedieved with another’s berachah.
Mishnah Berurah advises that it is proper to do this when there is concern that not everyone present knows the blessing, or remembers to make it, which explains why it is done in shuls and large communal gatherings.
The question of hearing a berachah acharonah from somebody else is the subject of a dispute among rishonim.
Shulchan Aruch (213:1) rules like the opinions of Rif and Rambam, according to whom bread and wine require haseiva (leaning together, thereby forming a united group) for one to hear the blessing of another, but for other foods it is sufficient to sit together. Shulchan Aruch adds that nowadays, it is not our custom to lean, so that sitting together helps for all foods. Rema disputes this, and writes (based on Raavad) that sitting together (or haseiva) does not help for foods other than wine and bread.
Fost factum, Shulchan Aruch (167:13) rules that in any event, collective intention to be yotzei helps. Magen Avraham (167:28; 113:4) disputes this, and rules like other rishonim who maintain that if the group does not sit together, one is not yotzei, yet sitting together helps bedieved for all foods. The Vilna Gaon rules like Shulchan Aruch, as does Mishnah Berurah (65).
Mishnah Berurah adds (213:9) that nowadays, when many are not particular about blessings, one may rely on this opinion lechatchilah, so that one makes a blessing for all. However, where possible, he writes that those who know the blessing should say it on their own (together with the person making the communal blessing).