Allowing user registrations on multi-site with domain mapping


Looking for some general advice, as I think I haven't go this bit right.

I'm testing Events right now and have an issue I can't get my head around.

With plugins like Events (or Marketpress) when you make a reservation (purchase) you can create an account - you get directed to the login page of that domain.

OK, then I click the 'register' link and get directed not to the mapped domain but to the multi-site master domain user registration page.

If I create a user, that works, but it creates a 'widowed' user, i.e. a user not attached to any site and thus has no permissions at all when it logs in.

I'm sure I have something wrong, as otherwise these plugins could never work could they? But what - where should I be looking?