The option on the right-hand sidebar has disappeared, and when you add non-members to Circles you’re no longer given the option to invite them as well.
Project lead Vic Gundotra has posted to Google+:
We’ve shut down invite mechanism for the night. Insane demand. We need to do this carefully, and in a controlled way. Thank you all for your interest!
For any who wish to leave, please remember you can always exit and take your data with you by using Google Takeout.
It’s your data, your relationships, your identity.
Our own posts on the subject have resulted in thousands of comments requesting invitations.
If you managed to get in before invites were shut off, make sure to follow The Next Web on Google+!