volunteer

a person working on a non-compulsory basis for an organisation without being paid

Also defined in

Regulation (EC) No 261/2004 of the European Parliament and of the Council of 11 February 2004 establishing common rules… 32004R0261, Article 2, point (k)
a person who has presented himself for boarding under the conditions laid down in Article 3(2) and responds positively to the air carrier's call for passengers prepared to surrender their reservation in exchange for benefits.
Council Regulation (EEC) No 295/91 of 4 February 1991 establishing common rules for a denied-boarding compensation… 31991R0295, Article 2, point (e)
a person who has: - a valid ticket, - a confirmed reservation, and - presented himself for check-in within the required time limit and as stipulated and who responds positively to the air carrier's call for passengers being prepared to surrender their confirmed reservation in exchange for…