This example demonstrates how to use the CollapsedNavPaneItemsSource property to show custom items in a group when NavBarControl is collapsed. To generate the items, bind CollapsedNavPaneItemsSource to a collection and set the CollapsedNavPaneItemsTemplate property to visualize items from the collection. To get the currently selected item in a collapsed group, use the CollapsedNavPaneSelectedItem property.
This approach can be used to create a similar NavBar as in Outlook.