Under the site settings > security, there is an option that says "Require Approval Before New Users Can Sign In?". Â The help says "If checked then when new users register on the site they will not be able to sign in until they are approved by a site administrator or a user in the Roles That Can Create Users roles (see the Permissions tab)." Â You could either do that, or have a role that admin needs to assign before they can see the specific content only visible by that role.
Is that what you are looking for?