Related Clues:

• Add to the register
• Become part of
• Fall in with
• Make connections
• Enroll in
• 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.)
• Travel with
• Loner inclined to join in
• Join up, sing on