Once you’ve created a course in Moodle, you can enrol students to it. Moodle supports a number of enrolment methods for students, including self-enrolment, manual enrolment, and bulk enrolment options.
Log in to the Moodle dashboard and select Site Home to view your course list.
I'm sorry. I thought of myself as an accomplished Moodler. Then I tried to add a student who has taken Basic and Intermediate to the Advanced course. No way!
Course administration > users > enrolled users > enroll users > enter user name in text box (but also check role at the top) > search > if successful, click the enrol button.