Legs Weight

Hi. There a lot of problems.

1). The mesh is not vertex painted, it will not morph. Dresses must be vertex painted with 2 colors;
2). Some vertices are affected by more than 4 vertex groups, meaning the extra one get deleted upon import;
3). Is the top layer of the skirt meant to be transparent? If not, then you should delete everything that the top layer covers;
4). You must export the mesh as thin from md and add thickness/inner geometry only in places where it is visible;
5). Do not use Decimate, it ruins everything. If you do not want to retopologize the mesh, at least model in quads and do not lower particle distance too much;
6). Do not join the body with the garment until you make the latter fully functional. Transferring weights to the body can mess up weights, for example, the legs are affected by skirt bones now.

You should watch a different tutorial and make something simple like a tank top to wrap your head around the process first. Do not watch tutorials where the author tells you to download Blender 2.7