by 1670s as an abbreviation of company in the business sense, indicating the partners in the firm whose names do not appear in its name. Hence and co. to indicate "the rest" of any group (1757).
in Latin, the form of com- "together, with" in compounds with stems beginning in vowels, h-, and gn-; see com-. Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words (co-worker) and Latin-derived words not beginning with vowels (codependent), including some already having it (co-conspirator).
1650s, "to select (someone) for a group or club by a vote of members," from Latin cooptare "to elect, to choose as a colleague or member of one's tribe," from assimilated form of com- "together" (see com-) + optare "choose" (see option (n.)). For some reason this defied the usual pattern of Latin-to-English adaptation, which should have yielded co-optate (which is attested from 1620s but now is rare or obsolete). Sense of "take over" is first recorded c. 1953. Related: Co-opted; co-opting.