Teens probably, but i do know of a few parents going who have read the book.
I guess it kind of depends on where you're going. I know that B&N is doing a Prom type thing, so that probably all teens. Borders is doing trivia, a costume contest (im going as Alice) and an Edward-v-Jacob this (TEAM EDWARD!), so that's probbably got a mix of 70% teens, 30% adults. BAM is just doing a release as far as i know, so parents and grandparents who have read the books might go there for a quieter release.
♥Graceful Little Alice♥