Members are trying to accept invitations to a board today and the emailed link is leaving them without any success. When we test drove the "add members" function on Monday the link directed one to sign-in or log-in and create an account. Has something changed?
I'm not sure if this is the issue that you're facing, but there is a bug in Trello that prevents people from accepting invitations to public boards.
(The fix for that will go out with the next release of Trello)