If you have not got an invite to Google+ yet, here is some good news. You no longer need an invite to get into Google+ now. Google has opened up the service so that everyone can join regardless of invitation.
So, if you want to join Google+ but have not got an invite yet, head over to plus.google.com and sign in. There is no indication as to if Google has opened the service to everyone or if it is just for a few hours like the invitation. So, you better head there quick if you want to get in.
You might want to check out the review of Google+ here.
Update: "Everyone" does not seem to be the right word. It seems like many are still getting the "Keep Me Posted" message. Check out this discussion here.