Hi Ian,
My feeling on this is not to use any form of priority mode on your camera. It will be all over the place in such a situation and will likely give you some undesirable/unexpected results if light levels are low. Instead use Manual mode on the camera and set your flash to TTL (not TTL BL). As suggested above, if you are using the flash on camera then try to swivel it and bounce it of a ceiling or wall - up and to the side of your subject is probably best or even behind you if necessary - rather then straight up into the ceiling. That way you will get a little more 'shape' to the light on your subjects. Watch out for what you bounce off though - a strong wall colour will tint the flash - but you may have little option there...
Now you need to think about the Manual exposure for the ambient constant lighting levels in the room - how do you want to expose for that..? It's often good to allow some ambient in the image too - to add depth to the image rather than an explosion of flash which obliterates all the ambient.
ISO800, f4 @ 1/60 is a rough starting point for incandescent lighting - I'd start around that and alter your shutter speed/iso accordingly. Underexposing the ambient will add more mood.. and slower shutter speeds will allow more ambient and 'atmosphere' especially with the DJ lights and so on...
If your use on camera flash in this way and you are moving around you will likely need to keep altering your flash compensation - don't be surprised to be all over the place on this -2.0 to +2.0 depending on the scene - Four chaps in black jackets filling the frame will likely cause the TTL flash metering to try and 'overexpose' the scene - so instinctively you might want to then set a negative flash compensation, and vice versa.
There is likely a mix of lighting colour temperatures to consider too - you might want to set your camera white balance to match the ambient in the room and now is the time to use the coloured gels which came with your SB-900! Set a tungsten white balance and use the amber gel for incandescent lighting, or use the green gel and set your camera for flourescent if that is dominant in the room.
Don't forget to shoot on RAW to give chance to alter white balance in post...
Once you are settled in, why not play around with slow shutter speeds ('dragging the shutter') with the DJ lights and experiment with first/second curtain synch...?
If you are shooting a group watch their front-to-back positioning. If you are around f4, you may find that depending on your focal length; even a step or two behind or in front of each other and they won't be in focus...
Lots to think about, but just some suggestions. Have fun!
Ash.
http://www.lightflow.co.uk