The gemara[1] cites a machloket regarding until which point birkat hazimun[2] extends. Rav Nachman says it is until “Nevarech…,” i.e., the introductory portion of the bentching said only when there is a zimun. Rav Sheshet says that it is until “HaZan …,” i.e., the end of the first beracha of Birkat HaMazon. Most Rishonim[3] understand that the machloket regards one who is answering but not bentching with the zimun. The question is, like yours, how long he has to take part in the zimun.
The Shulchan Aruch[4] accepts Rav Nachman’s opinion. Thus, Sephardic practice is to end the zimun at the conclusion of the zimun addition (“…u’v’tuvo chayinu”). In contrast, the Rama[5] accepts Rav Sheshet’s opinion.
The original intent of zimun was that only the mezamen[6] would bentch, while the others would listen silently and fulfill their mitzva through him. However, due to the concern that people would not concentrate sufficiently, the practice developed for the others to bentch silently along with the mezamen[7] or, if they have not yet finished eating, to bentch later. Although most mezamnim do not recite the whole bentching audibly, they should do so at least for the zimun, which, for Ashkenazim, includes HaZan.[8] Therefore, Ashkenazim should have the mezamen say the entire first beracha out loud.
When one is answering zimun but not bentching, he should listen to the zimun without eating.[9] Thus, in an Ashkenazi zimun, he should wait until after HaZan has been recited by the mezamen. For Sephardim, he can resume eating following the introductory part of the zimun.
It is interesting to inquire whether the need to wait is for the benefit of the one answering or whether it is an inherent requirement of the zimun as a whole. Two practical questions likely depend on this chakira.[10] One question is whether one who ate foods other than bread, and thus is not required to bentch and is not personally obligated in zimun, needs to wait until after HaZan before resuming his eating (for Ashkenazim). If he breaks his connection to the zimun after its initial stage, do we say that there was not a proper joining together to constitute a zimun? Another issue involves a joint Ashkenazi-Sephardi zimun. Can the Sephardi follow his own ruling and consider the zimun complete before the beginning of bentching, or does that ruin things for his Ashkenazi friends?
The Rosh[11] explains Rav Sheshet’s requirement to wait through HaZan as follows. HaZan is not part of the zimun. However, since zimun entails bentching together and the zimun is not a full beracha, it is not considered a zimun if the participants are not joined together for the first beracha of Birkat HaMazon. (Following this logic, several poskim say that when a zimun consists of ten men, at which time HaShem’s Name is mentioned in the zimun, turning the introductory section into a beracha of its own, it is unnecessary to wait until after HaZan.[12]) The Mishna Berura[13] explains that the first beracha is partially connected to the birkat hazimun. According to the latter explanation and probably according to the former, if there is not at least passive participation throughout the first beracha, an Ashkenazi should not consider it as though he was part of a complete zimun.[14]
Therefore, returning to the above questions, even one who did not eat bread should wait until after HaZan has been recited before continuing to eat.[15] Additionally, a Sephardi who interrupts his meal to participate in a zimun should wait until after HaZan to ensure that there is a zimun from the perspective of everyone involved. Yalkut Yosef[16] says that a Sephardi who leads Ashkenazim in zimun should recite HaZan audibly, and logic dictates that he would concur in our case as well. However, if bentching along with the zimun, Sephardim should not answer “amen” to the mezamen’s beracha.[17]