Volunteer
VOLUNTEE'R, noun A person who enters into military or other service of his own free will. In military affairs, volunteers enter into service voluntarily, but when in service they are subject to discipline and regulations like other soldiers. They sometimes serve gratuitously, but often receive a compensation.
VOLUNTEE'R, adjective Entering into service of free will; as volunteer companies.
VOLUNTEE'R, verb transitive To offer or bestow voluntarily, or without solicitation or compulsion; as, to volunteer one's services.
VOLUNTEE'R, verb intransitive To enter into any service of one's free will, without solicitation or compulsion. He volunteered in that undertaking.
[These verbs are in respectable use.]