join up

US /dʒɔɪn ʌp/
UK /dʒɔɪn ʌp/
"join up" picture
1.

to become a member of an organization, club, or group

:
He decided to join up with the local environmental group.
Many young people join up for various social causes.
2.

to join the armed forces

:
He decided to join up right after high school.
Many young men were encouraged to join up during the war.