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to join some organization. The club has opened its membership rolls again. Are you going to join up? I can't afford to join up.
to become a member of the military Levy joined up in World War II, received his citizenship and ended up a corporal in the combat engineers.
1. To become a member of some group or organization: After the recruiter's speech, I decided to join up.
2. To collaborate or team up with someone: The advertising firm joined up with the baseball team to promote the new manager.
3. To connect together: The points join up to form a straight line.
4. To connect something with some other thing: Join up the dots, and you'll see a picture. When we join these pieces up, the puzzle will be completed.