Eventually it worked, but not with the link in the email. Your friend needs to log into his/her account, and there accept the invitation (which I forgot where that is done). But beware, not all browsers will work (from an iphone it does not, it simply hangs), and it might be quite slow depending on the time and day.
But eventually, after countless tries and 2 emails to Diamond hotline, it worked. Quite embarrassing indeed.