You must be logged in as an Administrator to manage groups!
Steps
Navigate to the Manage groups page from the navigation bar.
You will be presented with this page:
Find the group you wish to manage either by searching, or stepping through the paged response. The results come back in alphabetical order.
Click on the Name of the group.
Click the User access link at the top of the page.
Find the user you are looking for either by searching or stepping through the paged response.
Click the Delete (cross) button to the right of the user you wish to remove:
Once all the required changes have been made, click Save changes in the top right of the page.