Thanks for reaching out to the community. When enrolling learners into learning groups, I don't believe you can specify the role they take in the course. Part of this reason this is the case is because the mechanics of learning groups relies on the individual being a certain role in the platform.
Hope this helps! If you are hoping to have folks as instructors in the course, you can do that within the course but not in the flow of learning groups. Let us know your use case and if this makes sense!