1
general::
Act together, combine efforts. For example, The public relations people joined forces to get better coverage for their candidates. This expression originally referred to combining military forces. [Mid1500s]
American Heritage Idioms
2
general::
Phrase(s): join forces (with someone)
Fig. to combine one’s efforts with someone else’s efforts. • The older boys joined forces with the younger ones to sing the school song. • Let’s join forces with the other faction and run our own slate of candidates.
McGrawhill's American Idioms And Phrasal Verbs