The Aruch HaShulchan has an opinion on whether one person can motzi (exempt) a crowd with a bracha acharona (after-bracha) on mezonot foods. It is along the lines of it is better not to have one person do this for everyone else. I saw it once, and now cannot find it. I have since heard it is "famous"! Does any one have a source?
He goes on to cover be-di-avad cases.