In the enrollment page, and with the user view, you can click on a user to see which groups and courses are linked to that user. You also have many options:
Link a group - Click on "Link Goup", select the group you want to link, and click on "Link"
Unlink group(s) - Select the group(s) that you want to unlink from that user, click on "Unlink group", and "Unlink"
Link course - Scroll down, click on "Link course", select all the courses you want to link to that user, and click on "Link"
Unlink course - Select all the courses you want to unlink, click on "Unlink course", then "Unlink"
Manage groups
Please note: You can also click on a group to see and edit which users and courses are linked to that group