volunteer

One who enters into, or offers themselves for, any service of their own free will, especially when done without pay.

The volunteers at the nature reserve meet up ever other Sunday to help its upkeep.

The largest age group for volunteers was 35-44, the CNCS survey said, and volunteers were most likely to be parents with children under 18. The typical volunteer may live in Utah, which ranked first in the percentage of people who said they volunteered among all 50 states and Washington, DC.

One who enters into military service voluntarily (but who, when in service, is subject to discipline and regulations like other soldiers), as opposed to a conscript.

A voluntary member of the organized militia of a country, as distinguished from a regular or member of the standing army.

That summer and fall the Army organized twenty-five regiments of volunteers, including two black regiments, which would have all-black captains and lieutenants. All officers were to be selected from regulars and volunteers who had distinguished themselves[…]

A person who acts out of their own will without a legal obligation, such as a donor.

A plant that grows spontaneously, without being cultivated on purpose; see volunteer plant in Wikipedia.

To enlist oneself as a volunteer.

To do or offer to do something voluntarily.

to volunteer for doing the dishes

Miranda: No good. Both routes are blocked. See these doors? The only way past is to get someone to open them from the other side. Shepard: It's not a fortress; there's got to be something. Here, maybe we can send someone in through this ventilation shaft. Jacob: Practically a suicide mission. I volunteer. Miranda: I appreciate the thought, Jacob, but you couldn't shut down the security systems in time. We need to send a tech expert.

To offer, usually unprompted.

to volunteer an explanation

To offer the services of (someone else) to do something.

My sister volunteered me to do the dishes.

To grow without human sowing or intentional cultivation.