Hi Sue

It is possible in v12 under the content tab to Show records by audience. If the course learners  could be added to an audience then only these users would show.  Unfortunately there isn't an audience rule to automatically select users by course enrolment (there is one for course completion)

It is possible to add users to audiences through bulk user actions. There is a filter for role and course category but not for course.

There is some documentation on modifying the report builder default values for filters code that may be helpful. Note the User report source doesn't include the course filed you probably need to use the course membership or course completion source to get access to course and user fields.

regards