How is “join up” used?

In Butch Cassidy and the Sundance Kid (1969), one line is: “But why would these guys join up and take after us, though?”

Which movie is “join up” from?

The clip at the top of this page is from Butch Cassidy and the Sundance Kid (1969).

But why would these guys join up and take after us, though?

8 作品で登場 · 1 本目

to become a member of the armed forces; to combine with sb else to do sth

6 本の映画に登場

1
to become a member of the armed forces
2
join up (with sb)
to combine with sb else to do sth
We'll join up with the other groups later.

定義：Oxford Advanced Learner's Dictionary 第 9 版