Hi Ali

This depends on how you are enrolling learners on the course - it possible for self enrolments, enrolments with keys, importing by CSV, or through enrolment plugins.

If you are getting the Trainer to assign the learner role on the course then you can change the Trainer role permissions to allow role assignment

Site Administration > Users > Permissions > define roles

Select the role and select allow for Assign roles to users.

Once this role has been modified you need to assign the modified training role to the person who is the trainer at a course level  by going into the course and going to Administration > Assign Roles

This would allow the course trainer to assign the learner role for just this course.

I can go over other options if this isn't how you enrol learners on courses.

Craig