You can set the permission for Manually Activate Users and Approve posts. If you want to grant this permission independently from other user roles, just setup a special user group 'New User Acceptor' (you don't need to call it that way ?) and add this group to the users in question.
Unfortunately (as far as I know) there is no way to hide user groups from showing up to other users. If that's important for you there is a way around this: assign a standard user group to everyone and create a second user group with the same name and appearance with this additional privilege. Only you, the admin, know the difference.