I remember reading in Blitz magazine an article by John Wills (Machado Jujitsu) where he demonstrates how BJJ can be used against multiple opponents. He was suggesting if you are taken down you go to the guard and use your opponent as a shield as you choke him out. Once he is out you try to trip or roll into another attacker and then do the same. Interesting strategy...
Another thing, a good takedown on a hard surface can do quite a bit of damage to your opponent.
John ismy BJJ coach (Will not Wills BTW). He tells a good story about a bar melee where he and one other guy went to the ground choked guys out and stayed underneath. They walked out unscathed. The other guys who stood and banged - while they won, came away with all sorts of cuts and bruises and injuries.