To detect a double-click on a GroupByRow, you could do something like this:

        private void ultraGrid1_DoubleClickRow(object sender, DoubleClickRowEventArgs e)
        {
            if (e.Row.IsGroupByRow)
            {
                // Do something. 
            }
        }

If you need to get more specific about which part of the GroupByRow was clicked, then you would need to use the MouseDown or MouseUp event and use the grid's UIElements to determine what was clicked.