If a host invites a guest to lead the Zimun before Birkat Ha'mazon, is he required to accept the invitation, or may he decline?
The Gemara in Masechet Berachot (55) lists three improprieties on account of which a person's life is shortened, Heaven forbid (listen to audio for precise citation): refusing an invitation to read from the Torah, refusing an invitation to lead the Zimun, and exerting control and authority over the public. Seemingly, then, a guest must not refuse the host's invitation to lead the Zimun, as he would thereby bring upon himself the severe punishment mentioned in the Gemara, Heaven forbid.
However, the Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), towards the ends of Parashat Shelah (24; listen to audio for precise citation), claims that this Halacha applied only at the time when the Mezamen (person leading the Zimun) recited Birkat Ha'mazon out loud on behalf of everybody at the table. Since nobody else recited Birkat Ha'mazon other than the Mezamen, a guest had to accept a host's invitation to lead the Zimun so that he could recite "Birkat Oreyah," the special blessing for the host that a guest adds in Birkat Ha'mazon. If the guest refuses, thus forcing the host to recite Birkat Ha'mazon, the host loses his blessing. There is a concept that a person who blesses others is himself worthy of blessing, and hence a guest who declines the opportunity to bless his host renders himself unworthy of blessing, and for this reason the Gemara warns that his life may be shortened, Heaven forbid.
Nowadays, however, as the Ben Ish Hai observes, the widespread practice is for all participants at a meal to recite Birkat Ha'mazon. Therefore, even if the guest refuses to lead the Zimun, he will still recite Birkat Ha'mazon and will thus have the opportunity to offer the special blessing for his host. Hence, the Gemara's warning towards those who refuse a host's invitation does not apply, and it is thus permissible for a guest to decline an invitation to lead the Zimun. This is the ruling as well of the Hesed Le'alafim (work of Halacha by Rabbi Eliezer Papo, Bulgaria, 1785-1828).
Of course, as the Ben Ish Hai emphasizes, a guest should certainly seize the opportunity to lead the Zimun, given the immense value and significance of this Mitzva. Although a person who refuses is not subject to the Gemara's warning, it is nevertheless proper not to refuse an invitation to lead the Zimun, and to rather eagerly seize this opportunity to perform a special Mitzva.
Summary: Strictly speaking, it is permissible for a guest to refuse a host's invitation to lead the Zimun; preferably, however, a guest should not refuse, given the value and significance of this Mitzva.
