This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. One who cooperates; an associate.

n. One who labors jointly with others to promote the same end.

n. ⟨ LL. cooperator, ⟨ cooperari, pp. cooperatus, work together: see coöperate.] One who acts, labors, or strives in conjunction with another or others for the promotion of a common end; specifically, a member of a coöperative society.

n. an associate in an activity or endeavor or sphere of common interest

"One of the main Arab American organizations in the country said that the word cooperator (ph) is a very negative connotation of the Arabic language, that it suggests something more akin to collaborator, someone whose siphoned out, and this is likely to hinder cooperation among Arab Americans in the program."

associate   

dancing partner    bridge partner