Here you are provided with a list of any existing users groups for the web site. On top is the group name, and below the line are the members of the group if there are any. Their User Name and Full Name is displayed.
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Clicking on the group name will take you to the Edit Groups window.
Clicking on the User Name will open the Edit User Groups window. There you will be able to edit which groups the user is a member of.
Click on the Back button at the bottom of the list to return to the Manage Users & Groups window.