a softbody would probably be great for this. I would model it in its resting state of being balloned out. Make it a soft body with a copy for a goal. I think thin you can fold up up the soft. When you animate it just take the coal weight from 0 up to something like 50 or more. this will expand it.. and should also let deformations happen on it.. like someones body pressed in to it. It will not be posible for it to push the bodys away buy its self though. Maya dons't have that kind of non-liners like that. You can just key the people going back or use a fild of some type that you put near the center of the bag, use it on the people. Also to add spice or realisom to the bag comeing out if your going to do some slow motion you can add some air fields and or turbulances to the bag's softbody as its unfolding.

I think this might work.

Good to see ya around Niobi