The main ground from the battery is bolted to one of the starter mounting bolts. The ground for the whole electrical system is bolted to the ignition coil mounting bolt.
If you were asking about the ground for the neutral light circuit, then Shadetree just told you in the post above.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.