I found the answer. It has to do with using Google Chrome and who it thinks it logged in. Even if I go to the gmail account of the user the invitation was sent to, if I had logged into another account first and not logged out of it (which I never really needed to do before), it was using the email address of the first account I logged in with. Once I logged out of all accounts, went back in to the one with the invitation, then it worked.
Tjesi