Shooting movement with the Canon 5D Mark 2

[wofsy]

The situation: 50 people dancing ecstatically to popular music in a room with mixed lighting - window light - tungsten ceiling lights. I am trying to catch individual dancers either in semi-head shots or body shots to show movement,joy, and gesture. Other figures can be blurred. In fact blur is good.

I am inside the crowd of dancers who are whirling around the room and changing pattern randomly.

I can use flash if I want. Without flash the room is often too dark to get a high enough shutter speed even at ISO 3200.

What is the best technique to get sharp images in this situation? I notice that the Mark2 has an AI servo mode which supposedly follows movement. How does one use this?

So far sharp images come by luck. Or one tries to lock on at rest points in the movement.

I have been using a 70-200 mm zoom for blur and reach.