If the enrollment key is set for the course, most of the students will enrol themselves. In some case, it might be helpful to manually add students or other participants to the course (i.e. non-editing teachers).
If you need to remove a user from your participants' list (i.e. a student dropped the course), you can manually do it:
If you need to remove multiple users, you can select them by ticking the checkbox on the left side and select Delete selected user enrolments from With selected users drop-down menu.