DataViews supports using the group footer as the group summary row.
Set the location option in the group footer settings to top or bottom to show the footer at the top or bottom.
In this demo, the group footer is at the top, and it adds together the data in each column in that group.
This also works with subgroups, and shows above the rows in the group when the group is expanded.
Try changing the footer location with the drop down menu above the grid and see how it changes the position when the groups are expanded.