Posing separates mesh (pic added)

So this model I am trying to pose come with meshes in different pieces. I use CTRL+J to join the mesh and it looks good… until I add the bones and try to pose it, then the mesh separates along the lines where it was originally separated. I am super new with blender and so I need some serious Noob help.

See, there at the waist and at the finger tips.