Source : Webster's Revised Unabridged Dictionary (1913)

Cooperator \Co*["o]p"er*a`tor\, n. [L.: cf. F. coop['e]rateur.]
   One who labors jointly with others to promote the same end.
   ``Co["o]perators with the truth.'' --Boyle.

Source : WordNet®

cooperator
     n : an associate who works with others toward a common goal;
         "partners in crime" [syn: {collaborator}, {partner}, {pardner}]