I'm sorry if this sounds harsh, but this is an incredibly stupid move. Right now is when you need to be opening the gates, while you have all of this publicity. If you close the doors like this and most of us only have a handful of friends, if that, people won't have a good experience, they'll say it sucks, and the site will be panned by critics before it even gets started.
This is exactly what happened to Wave. It sounded awesome, but nobody could join, so no one had any friends they could communicate with. I hoped you would have learned from your past mistakes. Social media sites are not the place to apply this silly semi-private beta thing that you use for the other Google services.