WordNet (r) 3.0 (2006):

unionize
    v 1: recruit for a union or organize into a union; "We don't
         allow people to come into our plant and try to unionize the
         workers" [syn: unionize, unionise]
    2: form or join a union; "The auto workers decided to unionize"
       [syn: unionize, unionise, organize, organise]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "unionize":
   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 forces, 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,
   unite with, wed