(n.) A mate, companion, or associate.

Bolshevik Bolshie Greek Red ace adjunct affiliate ally amigo associate auxiliary bedfellow bedmate belonger bosom buddy brother brother-in-arms buddy bunkie bunkmate butty camarade card-carrier card-carrying member cardholder chamberfellow charter member chum classmate clubber clubman clubwoman coadjutor cohort colleague comate commie committeeman companion company compatriot compeer confederate confrere consociate consort conventioneer conventioner conventionist copartner crony dues-paying member enlistee enrollee fellow fellow member fellow student fraternity man friend girl friend gossip guildsman honorary member initiate insider joiner life member mate member messmate old crony one of us pal pard pardner partner playfellow playmate pledge roommate running mate schoolfellow schoolmate shipmate side partner sidekick sister socius sorority girl sorority woman teammate workfellow yokefellow yokemate