Under the site settings > security, there is an option that says "Require Approval Before New Users Can Sign In?".  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)."  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?