In order to enroll/unenroll a student from a course, first access the course from your Portal. Make sure you are seeing it as a Staff user (if this is not the case please open a support ticket requesting this permission), and then click in the Instructor tab:
In there click in the Membership sub-tab and enter the students' email in the Batch Enrollment text box. You can add multiple emails separated by commas or new lines:
If Auto Enroll is enabled, students will be automatically added to the course, otherwise they will have to manually enroll themselves. It's often a good idea to leave it selected when enrolling students, but it should be unchecked if you are unenrolling them.
Click the button for the desired action (Enroll or Unenroll).
You can get a list of all enrolled users in a course from the Reports/Course Enrollments section in the Admin dashboard: