WordNet (r) 3.0 (2006):

in league
    adj 1: (usually followed by `with') united in effort as if in a
           league; "they found out that some policemen were in
           league with the criminals"

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "in league":
   affiliate, affiliated, allied, associate, associated, cabalistic,
   confederate, confederated, conspiratorial, corporate, coupled,
   enleagued, federate, federated, in cahoots, in collaboration,
   in collusion, in cooperation, in partnership, in with, leagued,
   married, paired, partners with, teamed, wed, wedded