OK, I'll try to give you a clear example. For the sake of simplicity, I'll only use 3 groups in my example. Imagine I am grouping a pile of blocks. For G1, I will group them by size, small and large. For G2, I will further group them by color. For G3, I would group them by shape.
The layout would look something like this:
G1a Large blocks
G1b Small blocks
G2a Large green blocks
G2b Large red blocks
G2c Small green blocks
G2d Small red blocks
G3a Large square green blocks
G3b Large rectangular green blocks
G3c Large square red blocks
G3d Large rectangular red blocks
G3e Small square green blocks
G3f Small rectangular green blocks
G3g Small square red blocks
G3h Small rectangular red blocks
Each of these are in a different section in the report. I want to keep the items grouped like they are (by size, color, shape) but move the field in section G3d to the position just below G2b. If I just drag it there, I lose the 3rd level of grouping.