Indeed, I've received your invitation twice so thank you for it but I guess they've locked the site with some sort of user limit

Goineasy9 wrote:

I guess it was a temporary opening because the invite icon is now gone off of the home screen in Google+. I'm glad I got Robin in last night, but, that was really by using an old invitation email. I'm glad we didn't delete it.
So, wait for the floodgates to open again, then use the old email to join. When I spot the icon again, I'll post.

I've read there having some privacy issues... I guess they'll open up again when they solve them

Regards

They closed the invites after 6 hours due to crazy demands. It's old news today.

Link to source here in Swedish:
http://www.idg.se/2.1085/1.394131/google-stoppar-inbjudningar-till-nya-facebook-utmanaren

Google translate:

On Monday, Google launched its latest attempt to challenge Facebook's dominance in the arena of social networking, Google + . A few invited guests were given the chance to test the new service. Google stressed that the service is still in an early stage, but on Wednesday released more users into by letting the existing invite their friends and they in turn had to invite her.

Many did, of course, test the service and it took just six hours before Google was forced to close function for invitations. The closure was due to "mad demand," according to Google's technology director, Vic Gudotra. Later he wrote on his personal Google +-page "We must do this carefully and in a controlled manner."

On Google's information page on Google +, it follows that for temporarily exceeded its capacity, but interested parties can submit their email address and be notified when it is ready to open for invitations again.

EDIT: http://xkcd.com/918/ from the comments :D