Very odd issue. I have a site in testing mode with a fresh installation of CBSubs...
www.girlology.com.php5-22.dfw1-2.websitetestlink.com/
When I installed in the CBSubs, I set up 4 subscription plans: 1 is private (admin) and three are public, which you can see on the site.
I was able to go into the backend and manually add three existing users to the admin subscription plan.
I then registered two new users, under two different plans. The users were not assigned to the plans, nor can they be added to the plans from the backend. The Subscriptions Tab in the backend has no options, and no subscription tab is available in the edit profile section.
The tab is however available for all three of the prior existing users.
I don't want to throw a major wrench into this because I'm pretty sure it's not affecting it, but I am running this site on Joomla MultiSites. But as far as I know, the users are not shared.