Synonyms for Collaborator (same or very similar meaning)
WordNet sense 1 (someone who assists in a plot):
confederate, henchman, partner in crimeWordNet sense 2 (someone who collaborates with an enemy occupying force):
quisling, collaborationistWordNet sense 3 (an associate who works with others toward a common goal):
cooperator, partner, pardnerFrom the ODE community, based on WordNetadd/edit