WordNet (r) 3.0 (2006):

team up
    v 1: form a team; "We teamed up for this new project" [syn:
         team, team up]

Moby Thesaurus II by Grady Ward, 1.0:

130 Moby Thesaurus words for "team up":
   act in concert, act together, act with, affiliate, affiliate with,
   align with, ally, amalgamate, associate, band, band together,
   be in cahoots, be in league, bracket, bunch, bunch up, cabal,
   cement a union, centralize, club, club together, coact, coalesce,
   collaborate, collude, combine, come into, come together, concert,
   concord, concur, confederate, conjugate, consociate, consolidate,
   conspire, cooperate, couple, couple up, creep in, do business with,
   double-harness, double-team, enlist, enroll, enter, federalize,
   federate, flock to, fuse, gang, gang up, get heads together,
   get into, get together, get together with, go along with,
   go in partners, go in partnership, go in with, go into,
   go partners, go with, hang together, harmonize, hold together,
   hook up, hook up with, join, join forces, join fortunes with,
   join hands with, join in, join together, join up, join up with,
   join with, keep pace with, keep together, league, league together,
   league with, line up with, make common cause, marry, match, mate,
   merge, organize, pair, pair off, partner, play ball, pull together,
   put heads together, rally round, range with, reciprocate,
   run parallel to, side with, sign on, sign up, sneak in, span,
   stand in with, stand together, stand up with, strike in with,
   string along with, swing in with, take out membership,
   take part with, take sides with, take up membership, team,
   team up with, team with, throw in together, throw in with, tie in,
   tie in with, tie up, tie up with, unionize, unite, unite efforts,
   unite with, wed, work together, yoke