I have developed a method of removing chronic pain and train people to be able to help others. When I train them I demonstrate on a "volunteer" member of the public. I also like to arrange one volunteer per trainee to work on at the end of the training. The thing is that it works with a 97% reduction of pain on average, so this isn't being a guinea pig or a test and when I promote it for free and ask for "volunteers" it doesn't get much response. I believe "volunteer" has a negative feel, like being asked to do something for no return.
Is there a better word I can use that suggests that it is a win-win situation, that they stand a very good chance of leaving pain-free and that the newly trained practitioner gets a lot out of working on their first client under supervision?