At my school, the fraternities do the serenading, but my chapter always sings back (I think we may be the only sorority here that does so). They only serenade during Monday night meetings, and we know in advance when they'll show up, so we can plan meeting accordingly - usually they serenade to introduce their new member class (in which case just the new brothers sing & dance), or to promote some kind of event coming up, like Derby Days. They often bring flowers to give to our president and some of the other sisters, too.
I really like the custom-- it's one of those things that I barely knew about before I decided to go Greek, and my first serenade, I was like, "seriously, this is REAL??" But I think it's really cute and fun, and most people seem to enjoy it.
