To make a user an instructor that can be scheduled by other users simply assign that user to an Instructor role. For more information see Adding Instructors,
A role that is not an Instructor role cannot be made an Instructor role. To create an Instructor Role, add a new role and check the box that says "This is an Instructor Role." If a user is assigned to any role that is an Instructor role, other users will be able to make reservations for that user as an Instructor.