Simplify the following text for a Grade 12 native English-speaking person.
Original: A voluntary association or union ( also sometimes called a voluntary organization , unincorporated association , or just an association ) is a group of individuals who enter into an agreement as volunteers to form a body ( or organization ) to accomplish a purpose .
Simplified: 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 .