Voluntary association

Group of people with shared interests or aims

A voluntary group or union (also sometimes called a voluntary organization, common-interest association, association, or society) is a group of individuals who enter into an agreement, usually as volunteers, to form a body (or organization) to accomplish a purpose. Common examples include trade associations, trade unions, learned societies, professional associations, service organizations, and environmental groups. All such associations reflect freedom of association in ultimate terms (members may choose whether to join or leave), although membership is not necessarily voluntary in the sense that one's employment may effectively require it via occupational closure.

From Wikipedia, under CC BY-SA. More on occurri.