Stefana,

I have take the code directly from the C# tab on the Infragistics Silverlight Sample Browser at: http://labs.infragistics.com/silverlight/lobsamples/2009.2/

The only differnce is that my code references my Infragistics datagrid with it's Group By functionality turned on

<

igGrid:XamWebGrid.GroupBySettings>

private void TestExcel_Click(object sender, RoutedEventArgs e)
        {

            Workbook dataWorkbook = new Workbook();
            Worksheet sheetOne = dataWorkbook.Worksheets.Add("Data Sheet");

            //Freeze header row
            sheetOne.DisplayOptions.PanesAreFrozen = true;
            sheetOne.DisplayOptions.FrozenPaneSettings.FrozenRows = 1;

            // Build Column List
            sheetOne.DefaultColumnWidth = 5000;
            sheetOne.Columns[1].Width = 8500;
            sheetOne.Columns[3].Width = 10000;
            int currentColumn = 0;
            foreach (Column column in this.dataGrid.Columns)
            {
                if (column.Visibility == Visibility.Visible)
                {
                    this.SetCellValue(sheetOne.Rows[0].Cells[currentColumn], column.HeaderText);
                    currentColumn++;
                }
            }

            // Export Data From Grid
            int currentRow = 1;
            foreach (Row row in this.dataGrid.Rows)
            {
                int currentCell = 0;
                foreach (Cell cell in row.Cells)
                {
                    if (cell.Column.Visibility == Visibility.Visible)
                    {
                        this.SetCellValue(sheetOne.Rows[currentRow].Cells[currentCell], cell.Value);
                        currentCell++;
                    }
                }
                currentRow++;
            }

            this.SaveExport(dataWorkbook);
        }

        private void SetCellValue(WorksheetCell cell, object value)
        {
            cell.Value = value;
            cell.CellFormat.ShrinkToFit = ExcelDefaultableBoolean.True;
            cell.CellFormat.VerticalAlignment = VerticalCellAlignment.Center;
            cell.CellFormat.Alignment = HorizontalCellAlignment.Center;
        }

        private void SaveExport(Workbook dataWorkbook)
        {
            SaveFileDialog dialog = new SaveFileDialog { Filter = "Excel files|*.xls", DefaultExt = "xls" };

            bool? showDialog = dialog.ShowDialog();
            if (showDialog == true)
            {
                using (Stream exportStream = dialog.OpenFile())
                {
                    dataWorkbook.Save(exportStream);
                    exportStream.Close();
                }

            }
        }

http://community.infragistics.com/forums/p/40720/227955.aspx#227955

Thanks,
George