Edit my 3d model and add vertices grouping

had a new mesh made for my fps arms and now they need vertex groupe to work properly. picture 1 is the first mesh arranged properly with the second mesh rotated to mach the original picture #2 is the new mesh imported it is backwards and should match the direction of the first . I cant figure out how to create a vertex groupe or modifier like the original model had. The third image is an example of the fbx setup i am using the arm mesh should distort when i move the bone inside my game engine . you can see the mesh warping when moved away from the bone. This should be a very easy project as all you neeed to do is copy the layout of the first set so that the new one will work properly. This was my first time using blender and i cant figure it out had a new mesh made for my fps arms and now they need vertex groupe to work properly. picture 1 is the first mesh arranged properly with the second mesh rotated to mach the original picture #2 is the new mesh imported it is backwards and should match the direction of the first . I cant figure out how to create a vertex groupe or modifier like the original model had. The third image is an example of the fbx setup i am using the arm mesh should distort when i move the bone inside my game engine . you can see the mesh warping when moved away from the bone. This should be a very easy project as all you neeed to do is copy the layout of the first set so that the new one will work properly. This was my first time using blender and i cant figure it out