A better word than "volunteer"

As suggested by Yosef Baskin, these people can be seen as participants in a free treatment program (or at least an opportunity to sample that program). They are not merely volunteers as they are as receiving something for free rather than merely donating their time. I suggest you frame your invitation to participate in these terms, treating them as applicants in order to suggest a service in demand:

We are now calling for applications to participate in the Laurel Program, offering potential relief from chronic pain, delivered free by our trainee practitioners. Applicants will be required to...