Multisite subsite independent registration

I use multisite and my main site is practicerecovery.com

I am teaching a course on family.practicerecovery.com and I need people to be able to register directly on that website without being re-routed to practicerecovery.com

essentially I need this to work:

family.practicerecovery.com/wp-login.php?action=register
without it redirecting to the
practicerecovery.com registration page.

I have researched this, which was not very hopeful:
http://wordpress.org/support/topic/multisite-and-user-registration

and this, which seems like a solution- but is WAY over my head:
http://halfelf.org/2012/multisite-registration/

There must be an easier solution!?

Help appreciated