Wikipedia
A voluntary group or union (also sometimes called a voluntary organization, common-interest association, or just an association) 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, and environmental groups.
Membership is not necessarily voluntary, as it may be effectively required in order to work, as is common with many teachers unions in the US. Because of this, some people use the term common-interest association to describe groups which form out of a common interest, although this term is not widely used or understood.
Voluntary associations may be incorporated or unincorporated; for example, in the US, unions gained additional powers by incorporating. In the UK, the terms Voluntary Association or Voluntary Organisation cover every type of group from a small local Residents' Association to large Associations (often Registered Charities) with multimillion- pound turnover that run large-scale business operations (often providing some kind of public service as subcontractors to government departments or local authorities).
Usage examples of "voluntary association".
That which was to be proved (namely, that government is a hallucination, or a self-fulfilling prophecy) could be shown only if all his equipment, including money and people, came to him through honest trade or voluntary association.
The most frequent case of voluntary association, next to marriage, is partnership in business: and it is not found or thought necessary to enact that in every partnership, one partner shall have entire control over the concern, and the others shall be bound to obey his orders.
Instead of tamely expecting their approach, the Azimuntines attacked, in frequent and successful sallies, the troops of the Huns, who gradually declined the dangerous neighborhood, rescued from their hands the spoil and the captives, and recruited their domestic force by the voluntary association of fugitives and deserters.
The bad old days were long past when law restricted voluntary association.
With his mind reverting to chess, Stephen said, 'Your republic, now, Mr Evans: do you look upon it as one and indivisible, or rather as a voluntary association of sovereign states?
They had armed themselves for the defense of their rights, had elected a Governor and a Legislature by voluntary association, had called a convention, and had adopted a constitution preparatory to admission to the Union.
Experience shows that for profound theoretical research in pure science, men work best when working alone or in a voluntary association of two or three brains, and when given the greatest possible freedom.
The wardroom theoretically was a voluntary association of officers, who could make their own choice as to what members they might admit.