To assign a user to a group, simply drag and drop the user into the desired group.  
You can then expand the group (1) to display which users are currently assigned to it.

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/134536/content">

To remove a user from a group, select the user (2) and then click the &quot;Remove User from Group&quot; button (3)