personally, i hate that kind of gift giving convention.
it's absolutely unfair of them not to mention it to you, and you'd be perfectly within your rights to say sorry, you didn't know. maybe bring a basket of goodies in after the holidays or something if you want.
if you have stuff to distribute fine, but there is no good reason to feel bad about telling people you didn't know about the mass gift exchange.
sorry. bit of a rant--the whole surprise gift thing is just one of my pet peeves.