Unfortunately, the GridView Data architecture does not allow the Empty groups to be shown in the ViewPort.
You can add group descriptors on the fly which can determine groups with no items, however the GridView will not display them.
The approach you can take with this scenario is to create your own abstraction over the grouping functionality and, create your own custom "grouping" depending on either row hierarchies or row details.
Hope this makes sense!
the Telerik team