There's a very big difference between A) being the only person in, with the ability to invite any of your friends in and B) everyone else already being in. Even if you could invite all your friends in, the only motivation for them to accept and actually join is you (not an entire network yet, until they all accept) which is never going to be enough motivation.
Currently those inside + at the moment cannot invite others. It will probably change soon, but I think the initial rollouts are still going on, so for the time being I cannot invite others.
Out of curiosity, do you know or have a hypothesis for why you were invited? It seems like there are a decent number of people here who have been invited, but many who have not, with a similar trend for #googleplus on twitter, and I've yet to discern a sensible pattern.
Friends of Google employees? That's how I got my invite...
My friend is a summer intern at Google (this year and last). He got myself and our entire Wave "crew" invited.