George S. Ledyard
I've trained in a couple of tactical ground defense systems but bar far the best training I've seen for defense against multiple attackers while on the ground is the Systema ground work. It's based on their fundamental principle of constant movement. It is the only system I have seen that would really help you much if you were on the ground in the middle of a group of folks trying to stomp you.
I've been a fan of using one person to shield me from his buddys by controling him from the backmount with his belly face up. Just hooks and a rear nakid choke, then he's mine to use as a shield until someone saves me.