If a class is full, you may still attend the course, and go to the first class session and ask the instructor if you may enroll. There is a 3 week enrollment period, so you may end up getting accepted into the course in proceeding weeks. We encourage you to continue attending until you are told it is completely full, or you are formally admitted.
In some courses, there is opportunity to be placed on a waitlist. If there is a wait list, the professor will refer you to the department office and advisor.