Wiktionary
vb. (context intransitive English) To enlist or enroll.
Usage examples of "join up".
We must wait and hide for twenty-four hours, till the other askaris, who are only a day's march behind, Join up with us, then we will attack.
Schofield was sent to North Carolina to join up with General Sherman, who was storming north to join the Army of the Potomac.
Once Karl had let a woman join up just because she had a thing for seeing slavers' blood.
As the plotters came up on Summerhall one by one, he defeated each of them in turn before they could join up with the others.
So, Smitt, if you are really willing to join up with us, keep that wish under mind blockĀ.
And in the end, a demon tried to eat me and then himself was after turning up in the flesh to bid me join up, as it were, and not a bit of good at all did it do to be telling him no then.