In order for instructors to view the roster, they'll need one of two permission sets assigned to them:
Option 1:
"Can view users" and
"Can view reservations"
These permissions let your instructors view the roster but not make edits. Just so you know, instructors will see all rosters and reservations, not just their own classes.
Option 2:
"Can view instructing classes"
"Can view users"
This will allow instructors to view only the roster for classes they are teaching, not any other classes. Instructors will also be able to view users' profiles belonging to locations they have access to.
Your instructors likely only have the "Can instruct classes" permission.