I'm also facing some problems with these invitations. A lot of them are receiving login not authorized errors when clicking in the "Join this portal" button from invitation email. I tried to add a test company to invite myself and I'm thinking that the problem happens when I create the user from project's user list, instead of the general user list (all users). I'm moving forward on this problem tomorrow and I will come back with more information if I achieve any success.
Thanks,
Renato A. Ferreira