Our experience of the last years has shown that many external users have been created multiple times (sometimes from different space owners).
Therefore we believe that it would be helpful if the sys admin had the option to specify that the username must be an email address (checkbox), or alternatively via a string field in which you can enter a username validation reggex string for field validation.
Another idea is, to search over all existing users (Full Name Field, "Forename Lastname") int he "create user" dialog and then "warn" the space admin if a user with the same name already exists.