unpaid
The office is staffed by unpaid volunteers.

full-time, part-time
committed, dedicated, enthusiastic, willing
qualified, trained
potential, prospective
conservation (BrE), hospital, Red Cross
church, community
local
healthy
human
parent, student

army, band, group, network, team
An army of volunteers cooked meals for the children.

appeal for, ask for, call for, look for
The charity is appealing for volunteers to take elderly patients to and from hospital.

find, get, recruit
We can't get any volunteers to help in the kitchen.

become
train
provide
The local community provided volunteers to repair the road.

could use (esp. AmE), need, want
work as

come forward (esp. BrE)
Hundreds of volunteers have come forward to offer their help.

man sth, staff sth
run sth
carry sth out
participate (in sth)
provide support, support sb
The support our volunteers provide cannot be measured in purely practical terms.

work
help do sth

staff, worker
labour/labor
group
coordinator, counsellor/counselor, driver, helper (BrE)
firefighter, fireman
organization
activity, effort, service, work
programme/program
vacation (AmE)

volunteer for
He worked as a volunteer for Oxfam.

army, civilian

recruit
serve as

army, corps, force, regiment, unit
soldier
military (AmE)