I think it would depend on why you are wanting to classify your role. Are you thinking about your resume or just what to tell other that you do? Speaking about a role is often easier since you have more time and words to use. If you are thinking about your resume, maybe don't actually title it. If you have a "Volunteer" section of your resume you can indicate the company name, years of service and a brief description, just as you would for a standard role (but minus the title).
Another option is using the word Volunteer. Volunteer isn't a bad way to describe yourself. You can also include a brief description of the ways you assist.
Keep in mind though, if the organization you volunteer with does have a specific name for its volunteers, use this. You can always elaborate.