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 a button on the Enrollment column, which allows you to enroll a user in that course. Click on "Enroll now" (1) to enroll this user onto 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 in the course you want.