I clicked the invitation link in the email I received. When I log in, it seemingly accepts the login credentials but brings me back to the login screen. My user id and password are correctly entered but have not been able to accept the invitation. The invitation, by the way, was sent to my work email address, which is the one I'm trying to log into the site with.
Sorry about that user and thanks for the report. I believe you've been a victim of a couple of our recent login and [invitation acceptance] bugs (14 - 7 = 3: Arithmetic overflow in Careers invitation?) which should both be fixed now. Take another crack at it and let us know if you're still having problems logging in or accepting your invitation.