To edit a user group, go to Profile in the lower section of the left navigation bar. Click on Your Organization, then on User Groups.
To add and remove members from a group, click on the blue expanding caret to the right of the group name.
On the next page, you may select new roles for group members and add new members to the group. You must use the Update User Group button before changes are in effect.
You may also delete the group altogether by using the Delete Group button. If you do so, you'll see this modal.
If you click Delete on the modal, the group will be deleted. It cannot be recovered. Any assets shared through the group will no longer be accessible to the former group members.