great way of training this is called the "Circle of Death". One guys is designated the "Coordinator", and one guy stands in the middle of a circle of people and closes his eyes. Each of the antagonists in the circle watches the Coordinator as they are assigned numbers from 1 - N.
Once everyone understands their numbers, the defender is told to open his eyes and prepare. The Coordinator then calls out one number at a time, in random sequence. The person with that number attacks, in any way they see fit, but with COMPELTE CONTROL. The defender takes that person on, and has, say 10 seconds to dispatch him.
After 10 seconds, another number is called, and so on, until everyone has had a chance to attack. The attackers can use any technique they wish, armed or unarmed, but the idea is speed, control, and non-compliance. The attacker means to give the defender a hard time, but must attack. No playing at the edges to use up time. Change defenders with each cycle so everyone has a chance to get the 'feel' for either side.
Over time, as the people involved become more comfortable with this mechanism, the 10 seconds is reduced. 8 seconds, then 5 seconds. Once you hit 5 secs, you change things up with multiple attackers.
First, one person attacks, and a couple of seconds or so later, another, with the Coordinator calling the numbers. 1-1, 2-1, 3-1. etc. With weapons, or without. Then decrease the number of seconds before the attackers pile up.
Eventually, you include Airsoft, with proper safety gear like masks and cups. Improvised weapons, movement, etc. Then add obstacles in the fight area. Furniture, walls, etc. Add E&E to the mix, so if the defender can get to a door and get out, he's safe. Etc. Lighting can be another way to manipulate the environment.
I've found this to be an outstanding way to train for multiple attackers and various scenarios. Goes great with Gabe's FoF ideas, as long as safety can be maintained.