Superuser invite

Fixes following fogbugz case http://manage.dimagi.com/default.asp?61877#351001 

Previously: Superuser can't be invited to join a new domain. Superuser gets "you are already a member of domain" message after accepting an invite for new domain.

With fix: Superuser can join any new domain with an invitation. 

At last, wrote a line of Python code, or half a line :)