tutorials can be found in http://www.book.artbeeweb.nl/?book=cloth-room

it looks like while gravity is pulling things down, the stretch forces are pushing / pulling the lower part of the pants outwards. Perhaps reducing forces of increasing density might make a difference, perhaps the wireframe view can reveal somewhat more too.

In the meantime, the persons legs should stop the pants from moving too far, assuming they are a proper collision target. And I don't see the relationship with Constraint Groups yet. The vertices in a cloth constraint group are 'forced' to follow the motions of the vertices of the object that they are constrained to, instead of the motions derived from the cloth-sim calculations. So it might make sense to constrain the waist band, and perhaps those leg-bands as well (although I don't see the need for that yet). 

Everything that's made of the same material can be put in the same (material) group.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though