Hi Peter,
Thank you for your interest in Syncfusion Products.
Please refer the online sample below which illustrates caption row summary calculated over all the records within a group and displayed in the caption cells.
http://samples.syncfusion.com/ASPNET/7.1.0.30/Web/grid.grouping.web/samples/3.5/Summary/SummaryInCaption/cs/summaryincaption.aspxPlease let me know if this helps you.
Thanks,
Rekha