Related Clues:

• Join up
• Register for classes (Var.)
• Register for
•     Join, with 'in'
• Recruit
• Add to the register
• Participate
• Join the club
• Sign up for a class
• Sign up, variantly
• Register for a class (Var.)
• Register for (with "in")
• Register, as for a course (Var.)
• Loner inclined to join in
• Join up, sing on