Hi Johan,
Thank you for your interest in Syncfusion Products.
You have to add the TreeGridHelper.cs file to the App_Code folder and TreeGridHelper.dll to your bin folder of your application to make this sample work. The TreeGridList class acts as a data source for the GridGroupingControl. This helper class looks up the tabular data and then exposes these records in an order that is conducive for tree like display. Please refer the sample in the below link which illustrates the above:
http://websamples.syncfusion.com/samples/Grid.Web/6.2.0.40/73119/main.htmPlease try this and let me know if this helps.
Thanks,
Melba