From Carleton Moodle Docs
Users can be assigned a role in a course without enrolling them in Administration > Course administration > Users > Other users.
This means they can carry out tasks relevant to their permissions but they don't appear in the Participants list. An example of this would be assigning a user to a Principal, Manager or Helpdesk type role to provide them access without confusing the participants list.
This page also lists users who have been assigned a similar role at Site or Category level. Carleton assigns site level roles for Helpdesk staff, Academic Technology staff and Library Reserves staff. Additionally, Academic Technology student assistants can create new course pages in certain categories, but cannot access the content of any courses into which they are not explicitly enrolled.
Note: By default, the only role which can be assigned to other users is the manager role. To enable other roles, such as teacher to be assigned, the capability moodle/course:view "View courses without participation" should be allowed for the role.