Hi Pontus,
GridGroupingControl maintains a DisplayElement collection in which each element represent a row in gridgrouping control and the element.GetRecord() method will return the corresponding record. Then you can use the SetValue method of that record object to assign values for them.
Please refer to this sample.
http://websamples.syncfusion.com/samples/Grid.Windows/F61588/main.htmBest Regards,
Saravanan