You need to use EditTemplate by editting it in Microsoft Expression Blend. Open the XAML that contains UXGridView grouping. Right Click UXGridView element>>Edit Additional Templates>>EditRowGroupHeaderStyle>>EditCopy. You can see that there are textblocks under RowGroupHeaderContainer. Then save the generate templates and open XAML code. Search for RowGroupHeaderContainer that you just generated.

<StackPanel x:Name="RowGroupHeaderContainer" Grid.Column="3" Margin="0,1,0,1" Orientation="Horizontal" Grid.Row="1" VerticalAlignment="Center">
	<TextBlock x:Name="PropertyNameElement" Margin="4,0,0,0" Visibility="{TemplateBinding PropertyNameVisibility}"/>
	<TextBlock Margin="4,0,0,0" Text="{Binding Name}"/>
	<TextBlock x:Name="AggregatesElement" Margin="4,0,0,0" Visibility="{TemplateBinding AggregatesVisibility}"/>
	<TextBlock x:Name="ItemCountElement" Margin="4,0,0,0" Visibility="{TemplateBinding ItemCountVisibility}"/>
</StackPanel>

You can discard the item count inside ItemCountElement. Or, you can set visibility collapsed and added your own TextBlock in there. Hope this helps.

Regards,
Handy