Users need to be able to register for a single private site?

I've got Pro Sites up and running and it looks very nice. However, my sites are somewhat different from the typical subscription blog/site. They're designed for extended families and each site will be paid for by only one person in the family. So Pro Sites works perfectly for that part. These sites are private, so I've also used the Multisite Privacy plugin to make them private.

Now for my problem: I need members of the paying client's family to be able to come and register for their family website, and theirs only. Right now, if I visit a private site (example: ) it encourages them to login, but if they click register, they're taken to the Network-wide registration form, and given the option to register for a new blog. If they sign up for an account only, it doesn't make them a member of the site they visited. I don't want that. I want them to register for this site only, and not have an option to get a new blog.

The new blog option should only be available if going through the pricing page and paying for a new family site. Regular users, should be able to register for an account, that gives them access to that one site, and nothing more. Any ideas on how to accomplish this? I found this plugin that sounds like it would do what I want, but it hasn't been updated in ages:

I'm trying to mimic the functionality I had on some single-site websites I built that do this nicely. But since they're single-site there's only one site for family members to register for. See an example of the kind of page I'd like to present them with, here: