How to keep vertex groups while joining a multi-object rigged model

I like to create my characters in Blender with lots of individual meshes instead of just one large one.
However, when I am finished, I need just one mesh, as I use the result for gaming.
The problem is that rigging and posing works fine, but then when I select all meshes and join them… all binding is gone. (I can re-assign manually, but it takes for ever)
I’m fairly new to all this, but I can see that the combined mesh has been arranged in vertex groups, so the information is still there…
Do you know a way of re-assigning the meshes vertex groups the way they where before joining all the meshes?
Or joining while keeping the binding?
Thanks a bunch!