1) When 10 people recite Birchat HaMazon and have a Zimmun, what should a person who did not eat with them answer? 2) Should one answer after ’Raboitai Nevoreich’ and/or any Amen after brachot?

1) He answers Yehi Shem Hashem etc.. 2) If a person hears a Zimun and did not eat anything with the group, he should respond to the leader: ברוך (אלהינו) ומבורך שמו תמיד לעולם ועד. After the leader has responded, he should answer Amen. He should also answer Amen after the Brachot.