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 ~ in communities.
= get together, link up
PHRASAL VERB: V P with n, pl-n V P