<DataGrid.GroupStyle>
                <GroupStyle>
                    <GroupStyle.HeaderTemplate>
                        <DataTemplate>
                            <StackPanel Margin="{Binding Path=ContentHorizontalOffset, 
                                RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type ScrollViewer}}, 
                                Converter={StaticResource MarginConverter}}">

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return new Thickness((double)value, 0, 0, 0);
        }