I created a new network with my free account, and invited one of my colleagues to be a member of it. He doesn't receive the invitation at all, but his e-mail address still vanishes from the pending invitations list!
I tried the same thing with a gmail account instead the corporate account, and the invited e-mail address vanishes from pending invitations list, although the invitation e-mail does arrive in this case.
Could the problem be related to the network auto-join feature? Or something else?