If two or more people wish to eat, and they are required to recite the same Beracha, it is preferable for one person to recite the Beracha for both himself and the other person. The rule of "Be’rob Am Hadrat Melech" establishes that public Misva performance brings greater glory to G-d than private performance of Misvot, and so it is preferable for a Beracha to be recited collectively, by one person representing a group, then for it to be recited individually.
However, a person who is not eating, and is thus not required to recite a Beracha, may not recite a Beracha on behalf of somebody who is eating and is required to recite a Beracha. If a person has no intention of eating, then his recitation of a Beracha over food is considered to have been recited in vain, and if people respond "Amen" to such a Beracha, their response is considered to have been in vain.
An exception to this rule is a case where one planned on eating, but after reciting the Beracha, he was unable to eat. An example would be one who recites "She’ha’kol" over a cup of water, but before he is able to sip the water, the cup falls and the water spills. Even though the person is unable to drink, nevertheless, the Beracha he recited is not considered to have been recited in vain. Therefore, if he had in mind when reciting the Beracha to fulfill the obligation for others who were also required to recite "She’ha’kol," they fulfill their obligation, even though the one who recited the Beracha did not end up drinking. Since the Beracha was perfectly valid at the time it was recited, those who listened to the Beracha fulfill their obligation through hearing this Beracha, and may then drink. This is the ruling of the Magen Abraham (Rav Abraham Gombiner, Poland, 1633-1683).
Different customs exist regarding the response of "Baruch Hu U’baruch Shemo" when one fulfills his obligation by listening to another’s recitation of a Beracha. Our community’s custom is not to respond "Baruch Hu U’baruch Shemo" after G-d’s Name when one listens to a Beracha in order to fulfill his requirement. Although one of course answers, "Amen" at the conclusion of the Beracha, he does not, according to our community’s practice, respond "Baruch Hu U’baruch Shemo" in such a case. This is contrast to the custom observed by Moroccan Jewish communities, who respond "Baruch Hu U’baruch Shemo" even when listening to a Beracha for the sake of fulfilling one’s obligation.
It should be noted that one fulfills his obligation by listening to another person’s Beracha even if he does not answer "Amen." Needless to say, one should answer "Amen" every time he hears a Beracha, but if, for whatever reason, one who listened to an entire Beracha with the intention to fulfill his requirement did not answer "Amen," he has nevertheless fulfilled his requirement.
Summary:
1) If two people plan on eating or drinking and are thus required to recite the same Beracha, one can recite the Beracha on behalf of them both, and this is actually preferable to each reciting his or her own Beracha. However, one cannot recite a Beracha on behalf of another unless he is also eating and thus needs to recite the same Beracha.
2) If two people require the same Beracha, and one recites the Beracha for both of them, but immediately afterward the food or beverage falls such that he cannot eat or drink, the other has nevertheless fulfilled his obligation, since the Beracha was valid.
3) One who wishes to fulfill his requirement by listening to a Beracha recited by somebody else fulfills his requirement even if, for whatever reason, he does not answer "Amen."
4) According to our community’s custom, the person listening to a Beracha for the purpose of fulfilling his obligation does not respond "Baruch Hu U’baruch Shemo."
