Western civilization has been (and is) characterized by voluntary associations of people, from the earliest warrior bands and "churches" to towns and universities and guilds, etc. Corporations, as vehicles for such associations, did not exist until relatively recently, and associations were (and very many still are) unincorporated. Unincorporated associations as a form of organization have been losing ground to the corporation, but are far from obsolete
Howard L. Oleck,
Nonprofit Unincorporated Associations,
21 Clev. St. L. Rev.
available at http://engagedscholarship.csuohio.edu/clevstlrev/vol21/iss3/4