Enrolment is different from authentication. In authentication, you confirm a user's membership to your site. In enrolment, you grant or confirm a user's access to a course. That is, authentication answers the question, 'Are you a member of this site?' Enrolment answers the question, 'Are you enrolled in this course?'

You have several options for managing student enrolment. They are found under Administration | Courses | Enrolments:

Notice the column labeled Default. At first, you would think this is for choosing the default enrolment method. But, you can choose only one of three methods as the default. The other five methods are not available. If this seems curious, here's why: Default is not for choosing the default enrolment method. It is for choosing the default interactive enrolment method. Interactive means that the student can enrol himself or herself into a course. Authorize.net Payment Gateway, Internal Enrolment, and PayPal are the only interactive methods in that...