Collins COBUILD Advanced Learner's English Dictionary - join up

1. If someone joins up, they become a member of the army, the navy, or the air force. When hostilities broke out he returned to England and joined up. = enlist PHRASAL VERB: V P 2. If one person or organization joins up with another, they start doing something together. Councils are joining up with their European counterparts... They began to join up in communities. = get together, link up PHRASAL VERB: V P with n, pl-n V P