join all meshgroups of this dress and now to transfer weights you need to click on your reference mesh, then hold shift and select your mesh.
Choose reference mesh similar to your (long dress) and append it in blender. After thay you need to redo weights, they are really messed up, I do not know why but you need to retransfer them. This tutorial is not best, but as you read it you'll see how to delete body parts. cause they'll glith with dress while moving. But before you import your mesh you need to delete body parts under your dress. Then s4studio_mesh_2 should have cut number 0000. As a base use Lace lingerie underwear fullbody and delete s4studio_mesh_1 and s4studio_mesh_3. Study some ea dresses and see how they are divided and why).
I think it'll be better for you to work only with one meshgroup (this is definitely not how meshgroups should be divided, they are three for some reason.
