WordNet (r) 3.0 (2006):

coconspirator
    n 1: a member of a conspiracy [syn: conspirator,
         coconspirator, plotter, machinator]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "coconspirator":
   Machiavellian, abettor, accessory, accomplice, accomplice in crime,
   cohort, confederate, conniver, conspirator, conspirer,
   counterplotter, exploiter, fellow conspirator, finagler, intrigant,
   intriguer, machinator, maneuverer, operator, opportunist, plotter,
   schemer, socius criminis, wire-puller