In order to enroll a user in a course, visit this course and go to the tab Users. Over there you will be able to view a tabular list of all users in the system.
You will notice on the Enrollement column, a button which allows you to enroll this user to that course. Click on "Enroll now" (1) to enroll a user in this course.
Alternatively, you can visit the user's profile and over there navigate to the Courses tab. Over there you will be able to view a tabular list of all available courses.
Again click on "Enroll now" (1) to enroll the user to the course you want.