1. Select the clothes mesh
  2. Select the root bone of the rig
  3. Go to SKIN/BIND SKIN (Keep Default Parameters)
Default Parameters
  1. Select the skinned body mesh
  2. Then Select the clothes mesh
  3. Go to SKIN/COPY SKIN WEIGHTS
  4. Select Closest Joint
  5. One to One
  6. And Normalize

Press copy.