The grouping can help, but your primary problem is the Joint Parameters as trav suggests above.  Except that he left out the Outter falloff zone... here's the deal:

• Everything inside the green sphere (of that body-part, or it's direct parent) gets affected.

• Everything outside of the red sphere is completely not affected (except for children of that part).

• The polygons between the green and red spheres (the 'falloff zone') are affected less and less, the closer they are to the red sphere (outward from the green sphere).

• All of the above is over-ridden and/or constrained by the Joint Angles, where similar rules apply (everything between the green bars moves intact, everything outside the red bars is not affected, everything between the green and red bars is the deforming area).

• Always keep in mind:  Children can affect their parents, but not thier grandparents or siblings.

...so, you should start by setting up the Joint Angles... use the Front Camera (or make sure your camera is not rotated) and drag the bars around so they're in the right place - in particular, the red bar that points down through the crotch area - it should not overlap/encompass anymore polygons of the hip than you want to be affected, but it should be far enough to get the ones you do want affected (I suspect that this might be your current problem).  If the angle needs to be wide to include some of the crotch polygons but that angle also causes includes some other polgons that you don't want affected then...

Once the bars are in the right place, you can use the spherical falloff zones (red and green spheres) to further limit and refine the deforming area.

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