Currently there is no way to sort group rows in RadGridView.
I am considering replacing Syncfusion with Telerik in my application but this issue is a showstopper. Syncfusion's grouping grid can do this and I use it heavily in my application.
Our need is to aggregate the group data showing group count or sum of an amount, and allow the groups to be sorted by either the count or the amount so that work can be prioritized.