If your "free plan" is lifetime free plan, and in general CBSubs settings you have:
- Allow Free Registered Users (without subscriptions) YES
- Create Subscriptions also for free lifetime plans NO
Then the users are "auto-assigned" to it.
Otherwise you can use Community Builder -> Plugins -> CBSubs -> Import
And there choose "Import users from a given CMS ACL", and choose the Joomla ACL group of your regular users (e.g. "Registered").