noun
BAD : The hospital could not operate without voluntaries.
GOOD : The hospital could not operate without voluntary helpers.
BAD : In my opinion, these young voluntaries should be congratulated.
GOOD : In my opinion, these young volunteers should be congratulated.
◆◆◆
Voluntary is an adjective (NOT a noun). It describes someone who agrees to work without being paid, or work that is not paid: 'On Saturdays she does voluntary work at an old people's home.'
The noun is volunteer = someone who decides to do something when they are not forced to do it: 'I need three volunteers to help me move the piano.'