WordNet (r) 3.0 (2006):

join forces
    v 1: work together on a common enterprise of project; "The
         soprano and the pianist did not get together very well";
         "We joined forces with another research group" [syn:
         collaborate, join forces, cooperate, get together]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "join forces":
   affiliate, ally, associate, band together, be in cahoots, bunch,
   bunch up, cabal, cement a union, centralize, club, club together,
   come together, confederate, consociate, conspire, couple,
   federalize, federate, gang, gang up, go in partners,
   go in partnership, hook up with, join fortunes with, join together,
   join up with, join with, league, marry, organize, pair, pair off,
   partner, stand together, stand up with, team up with, team with,
   throw in with, tie in with, tie up with, unionize, unite with,
   wed