Initially it was open but then they closed it when too many were signing up. Invitation will limit the inflow numbers without restricting it altogether.
I don't see lasting long as its nothing revolutionary compared to Facebook. Reason why Facebook is where it is today is the ingenious brains of Zuckerberg and Co. Things like status updates, writing on friends wall, exclusivity of networks and above all the lack of junk flashy ads is what set Facebook apart from Hi5, Friendster and Myspace.
So unless Google does something extraordinary to outgun Facebook, its a goner already. So far the Circles are a big + but heck, Facebook can put that up easily.
But agree this whole "invite" business is annoying. Obviously they haven't learnt from Wave. Why not release a complete polished product instead of half done buggy stuff? If Facebook can, why can't they? It reflects badly on Google in my opinion, a company known for having a teams of top notch engineers behind its products.