The word volunteer is a term that describes someone who is carrying out a task for the benefit of others and does so unpaid. In sport, some people choose to call themselves volunteers, some say they help out and some call themselves by the name of their role (coach, club captain, treasurer). The language used isn’t as important as making sure the person doing the task is having a fulfilling experience.
By definition, a volunteer is someone who gives up their time to benefit the environment, individuals or groups other than, or in addition to, close relatives without being paid for it.
Volunteers are under no obligation to do the work, it’s a free choice to volunteer. You may be paid expenses but these can only reimburse you for costs incurred whilst you were volunteering, like food and travel.