 It may be possible for students to self-enroll in a course. They click the link to the course, the button that reads Enroll Me, and they are enrolled. Alternatively, it may be preferred that teachers manually enroll students in their course. So go to the course, down to the settings block, Users, Enrolled Users. Click the button that reads Enroll a Users, and then select the user or users from the list and assign the role.