How do I add individual teacher/student/non-editing teacher/observer to a course?

If you are permitted to add students to your course this is how you proceed:

1. From the course administration block click users/enrolled users.
2. Click enroll users button top right.
3. Search by email address for user you would like to enroll.
4. Make sure the correct role appears at the drop down at the top.
5. Click enroll button and finish enrolling users. 

The user will now be enrolled in the role you selected in your course.