I got it to work, but with a braindead manner. Structure is this:
Group 1
Details
Now, Group 1 is sort of "customers" and details contain their "orders" (which is not exactly the case here, but something like this). One page per customer etc.. Now, in the detail section I have a text box with a switch function that creates numeric values based on a certain column in "orders". Then to get a sum at Group 1 footer, in the detail, I use an invisible text box that has Running Sum Over Group set and source as the switch text box, and then refer to this box at Group 1 footer. Puhhhleeeeeze.
How would this be done the right way?