I've never seen any rating that mentioned multiple wires in each hole. If you've run out of space for ground wires, you can get add a lugs from an electrical supplier.
And Alectrician is right, the netural and ground are bonded together at the main panel but not at any sub panels to avoid return current from sub panels from using the ground as a neutral return path. But around here, the grounds are all terminated on the grounding bus bar and the neutrals are all terminated on the neutral bus bar but the neutral bus bar is just bonded to the panel with an optional screws that's included with the panel.