Course Enrollment
Before a learner can access your course, they must first be authenticated on the site and that is the responsibility of the site administrator.
There are several enrollment methods available to a teacher, what is available, again, depends on the site administrator. The admin might enroll students into the course automatically, or the teacher might be able to let students enroll themselves. This is the manual enrollment method. Other options can be seen in the Enrolment methods link and include Self-enrolment and guest access.
Note: Guests can only view the course and its resources; they cannot participate in any activities.
An enrollment key may be set if self-enrolment is enabled so only students with the key can enter. The screencast Giving learners course access gives more information on course enrollment.