There are two issues . 1. keeping the waist of the skirt in place and 2, keeping the cloth stretching under control

keeping the skirt in place can be done by constrained group or coreographed group. The constrained group only works when the vertex is in collision with the figure. Choreographed simply doesnt move unless you move it , its meant for animating the cloth but works to keep it in place .
just a few vertices are needed, maybe 1 row at the top of a skirt.

the cloth stretching is harder and there are several parameters that affect it , as well as the structure of your skirts geometry. Read the manual and there are tutorials on the cloth toom.

Manual is in the Poser folder as a pdf file .