In this task, we ask you to elaborate the sentence without changing its general meaning. You can do so by explaining further the input sentence, using more precise wording, adding qualifiers and auxiliary information etc.
Q: A voluntary association or union is a group of individuals who voluntarily enter into an agreement to form a body ( or organization ) to work together for a purpose .
A: 
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 .