If you like clowning around but you're not quite ready for the circus, you could enroll in clown college to perfect your skills. When you enroll, that means you formally add your name to the list — or roll — of students.
You can enroll in things other than schools or classes, and you can also enroll someone else. You might enroll your Chihuahua in a research project investigating whether dogs like to wear hats, or you could enroll your rude nephew in charm school. The "roll" of enroll goes back to the rolls of parchment that were used in medieval times for writing official documents.