Hi Sorin,
Sorry for the delay in responding you.
In GridGroupingWeb we can create relationship between the tables and display it in the hierarchy mode. Currently our code library does not support your requirement. Please refer the sample from the below link, which illustrates of creating related tables in GridGroupingControl:
http://www2.syncfusion.com/samplebrowser/samples.aspxPlease let me know if you have any other concerns.
Regards,
Raji