Try this one:
UltraGridColumn columnToSummarize = ugSelectStock.Rows.Band.Columns["quantity"];
SummarySettings ss = ugSelectStock.Rows.Band.Summaries.Add("Grand Total", SummaryType.Sum, columnToSummarize);
ss.Appearance.TextHAlign = Infragistics.Win.HAlign.Right;
ss.Appearance.FontData.Bold = Infragistics.Win.DefaultableBoolean.True;