Reuven makes hamotzi at the Shabbos table on behalf of the oilam. Shimon is sitting and waiting to be passed a piece of challah by the baal habayis. Levi comes late and makes his own hamotzi, having in mind only himself. May Shimon answer Amein, even though he has yet to eat the challah? Would it make a difference whether or not Reuven has already tasted the challah?
Shimon should not answer Amen.