If you want to allow certain (authorised) users to be able to log in to the front-end, but not allow regular site visitors to be able to register themselves, do this:
- disable user registration by changing the Allow User Registration field in Global Configuration to No. To learn how to do this see changing user registration settings.
- enable the Login Form module and ensure that it is visible somewhere on your website. To learn how to do this see configuring the Login Form module.