Google has closed the invites for Google+ for the night, According to an article posted on StateOfSearch.com.
They did this with Google Wave when that first came out and knew that they would probably have to do it for Google+ as well.
Don’t worry though, most sources think that invites will be opened up again tomorrow once the queues get sorted out.
Once you’re in, check out my article on How to Invite Friends to Google Plus.
See you on the inside!