Selective user registration using django_registration?
You said you have a database of club members already, so you must have a primary key or a tuple  which is unique for that database like a club registration number
which club members should know already

Tell Users to give there primary key value(club registration number) at the time of registration.
Make that club registration number also as a primary key to the new database which you  are creating after registration for the user, next time if some body will re-use that club registration number to re-register then It will fail as the database tuple will already be there associated with that club registration number
Also have a warning message at the time of registration that you can use one club registration number for one registration only for the site.
I have a site catering to multiple clubs, and each club has administrators who maintain a database of club members.  I want to limit site registration only to members who have explicitly been added to the club's database.  How do I go about auto-generating and sending out registration links to members as they are added to the database?  In other words, I want registration to be initiated only by club administrators.