Subtotals in a group footer?

Is it possible to display the subtotals by attaching a footer to the group? Instead of in the group header?

No, it not possible.
As a work around you can use sub grid functionality.

Hi,

Is there any sample using sub grid to show subtotals in group footer? I found this http://dhtmlx.com/docs/products/dhtmlxGrid/samples/07_extended_modes/03_pro_subgrids.html

But it will include grid header and it will not align with main grid columns.

Unfortunately such a solution won’t work well for your needs.
As it is not available to align your subgrid cells with the cells of the main grid.
Also your groupping most probably will break in case of sorting or filtering of the original grid.