If this is the case do not create a signup form on your site just a login page so that signup is not allowed for public only login and populate the login table yourself on the backend so that only people that you have 'signed up' are able to login.
Or as stated earlier use access levels or account activation. The user can signup but can not login until the account has been activated by the administrator. Or users can signup and login but can not see pages restricted to higher access level until administrator sets their access level permission to view pages restricted to their access level.