A volunteer was an individual who offered to do a task or job without being forced to do the service. The process of becoming a volunteer was known as volunteering.
In 2254, a landing party of Spock, Garrison, Una Chin-Riley, J.M. Colt, and two others volunteered to rescue Captain Christopher Pike from the Talosian community. They knew the situation and were willing to face the risk of finding themselves beamed into solid rock if the measurements and readings were an illusion created by the Talosians. If any volunteer wished to back out, Una informed them that nothing would be said of the matter. None accepted the offer. (TOS: "The Cage")
In 2378, Neelix, who was good with children, volunteered to be a mentor to Q Junior. (VOY: "Q2")