I'm contemplating a couple of options: A) Create a lot of buzz, just before the opening B) Create quite a bit of buzz about the beta phase and have interested people sign up/test C) Just let the 'community' do its work, after all, what's a community when your friends are not on it.
I've seen sites use option A and they really do get 10.000 users within a month, if not less, but are these the type of users you want for a community?