Subdomains' user registration and Pro Sites

Here is my question/problem... I was interested in trying Pro Sites sometime in the future...however I am also trying to solve the issue of my subdomains' users registering through the main site. So I am wondering whether Pro Sites would solve this problem out of the box...ie with Pro Sites, do the subdomains have their own, separate from the main site, registration form? I can't imagine your paying customers would gladly share their users with the main site or anyone else on the network so I am hoping that this is an option...
Thank you in advance,
Betty