Here you can add or remove user from roles. First, you need to select the site (since roles are always connected with a single website) and then check the appropriate boxes and click OK.

Please note: if you cannot see any sites in the listbox, you may need to assign the user to some site on the Sites tab.

Page url: http://devnet.kentico.com/docs/5_5r2/contexthelp/index.html?roles_tab.htm