From the navigation bar, find "Enrollment". Here you will manage enrollments of classes in courses, and link users to classes. The features will change depending on which view you choose at the top right corner:
On the top right corner, click on "View" next to switch view.
Here you have 2 options: Groups and users
Users view:
Link user - Click on the icon on the right of the user you want to link to a group, select the groups you want this user to be linked to, then click on "Link"
Assign group - Select all the users you want to link to a/many group(s), click on "Assign group", select the groups you want the users to be assigned to, then click on "Link"
You can also click on a user to assign a group and course to it