I still don't understand these G+ invite threads, mainly cause i was already invited and no mail arived in my inbox neither G+ let's me get into their website...
With or without invitation all you see is this page
https://plus.google.com/You guys are clicking on the wrong link. Don't click the big Learn More button in the email. Click the view this thread on google+ or whatever it says right above the actual message that gets sent out. It will then let you sign up that way.