You shouldn't have to change anything. The default behavior allows anyone assigned the teacher (or higher) role for a particular course to access the attendee list of all sessions within that course.
If you've done any local customization I suggest going back to look at those and see if one of those somehow broke the original functionality.
As far as I know there isn't a way to restrict access to particular sessions within a given course - all the settings are course-wide and course-specific.