You can try to use RowGroupHeaderTemplateSelector property which enable developer to gets or sets the template selector that is used when rendering the row group headers. Please refer to the sample of Row Group Header Template Selector in ClientUI.Samples project or check the online version at http://live.clientui.com/Default.html#/UXGridView/Styling/RowGroupHeaderTemplateSelector.

This sample shows how to customize the UXGridView’s row group header template based on custom logic through the RowGroupHeaderTemplateSelector property. In this sample, when the data is grouped base on DateIn column, user will see additional information about how many leave requests is approved and disapproved.

The other option is to remove/hide the UXGridView RowGroupHeader’s item count and then add another TextBlock element to display the desired information. For more information about how to hide the UXGridView RowGroupHeader’s item count, please check the solution in this thread.

Hope this helps.