<CollectionViewSource
        x:Name="groupedItemsViewSource"
        Source="{Binding Groups}"
        IsSourceGrouped="true"
        ItemsPath="TopItems"
        d:Source="{Binding AllGroups, Source={d:DesignInstance Type=data:SampleDataSource, IsDesignTimeCreatable=True}}"/>

<GridView.GroupStyle>
    <GroupStyle HeaderTemplate="{StaticResource MyGroupHeaderTemplate}"
                Panel="{StaticResource MyGroupItemsPanelTemplate}"
                HidesIfEmpty="True" />
</GridView.GroupStyle>