Hello Frustrated Dev,
The property you need to set is RadGridView.GroupHeaderTemplate
this is a data template , where you can put any user controls. The DataContext is a GroupViewModel which gives you access to any needed properties of the group ( including aggregate results) .
If you find much troubles implementing this please feel free to send me your project with the requirements about the contents and behavior of the group header and I will gladly make the modifications for you.
the Telerik team
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