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