This should have been posted as a support question, not a review. In the future, questions and issues that are support related should be posted in the support forum, not as reviews.
To answer your specific question, the plugin operates under the premise that users are “users” – meaning they have login credentials to access content. That doesn’t mean they have to register themselves. You can certainly remove the registration form in the plugin’s settings and/or not use a specific registration form. But in that manner, you would be creating the users yourself (via Users > Add New).
As @incareof mentioned, the plugin has moderated registration that can be used to require users be approved by an admin prior to having access. That’s one option.
Another option is to implement the Invite Codes premium extension. The primary feature of the Invite Codes extension is to require that the user have a valid invitation code (generated by you) in order to register. Without a valid code, the registration cannot continue.