What is another word for "volunteer"?

Roget's Thesaurus (1911) lists these synonyms for "volunteer": about, accord, activity, adventure, all, and, anvil, apprentice, authority, begin, beginning, betake, bid, break, bribe, broach, business, candidate, candidature, cause, choice, choose, come, compact, conation, contract, course, desire, determination, determine, devote, discretion, disengaged, embark, emprise, engage, engagement, enjoin, enterprise, execution.