volontulo

The build is volont + ul + o, and getting that right is the whole lesson. -Ul- is the suffix for a person characterised by whatever precedes it: junulo a youngster, riĉulo a rich person, sanktulo a saint. So a volontulo is a willing one - somebody who acts volonte, of their own accord and without payment. The Reta Vortaro files it under the root volont-, alongside volonte, volonteco, volonti and volontuli, and that is the authority for treating volont- as one piece.

Here is the trap, and it is the best one in this band. Esperanto has a future active participle -ont-, and it has the root vol-, to want. Stack them: vol + ont + ul + o. That spells volontulo, letter for letter, and it parses perfectly - "a person who is going to want". It is grammatical, it is decodable without a dictionary, and it is not what the word means. Two different seams, one spelling, one correct answer.

How to tell which parse is live. The same way as always: ask what the meaning comes out as. "One who is going to want" describes nobody in particular and certainly not the person handing out programmes at a congress. "A willing one" describes exactly them. The pieces of an Esperanto word are invisible, so the seam you choose is the seam that makes sense - and where two seams both parse, only one of them will mean anything.

A footnote on the -ont- participle worth having. Nothing above is a criticism of -ont-; it is a real and useful suffix, and la venonta semajno (the coming week) is everyday Esperanto. The point is narrower: a participle suffix embedded in a borrowed root is not that suffix. Compare momento, where the -ent- is equally inert.

The rest of the family. Volontuli is to work as a volunteer, volontulado is voluntary work as an activity, and volontula is the adjective. The Esperanto movement runs almost entirely on volontuloj, which is why the word shows up as early as it does in the corpus.