I played DnD 3.5 while in the military and due to a shortage of DMs, was constantly flooded by players. Some of my games would include up to 10 or 11. I discovered quickly that balancing for this in 3.5 was neigh impossible: either the enemies would die before they could react, or the players were so over matched they could not hit them at all.
I tried horde style fights, big boss fights, etc, and generally it resulted in either massacre of the opponents, or TPK with nothing really being a satisfactory fight. I even would double or triple the HP of enemies to make it work.
Most game systems have a breaking point. How do you tweak systems to be accommodating for groups that far above the recommended limit?
Note: Simply telling people they can't join, or running two campaigns is simply not an option given the situation.