Hi Bill,
Thank you for posting query to us.
You can try the ListBoxSelectionMode in the help index to get details about the NameSpace and Assembly.
Namespace: Syncfusion.Windows.Forms.Grid.Grouping
Assembly: Syncfusion.Grid.Grouping.Windows (in Syncfusion.Grid.Grouping.Windows.dll)
Please refer the sample in the below link which shows the ListBoxSelectionMode in GridGroupingControl.
http://websamples.syncfusion.com/samples/Grid.Windows/F66658/main.htmThank you for using Syncfusion Products.
Best Regards,
Jeba.