Hey,
I do quite a bit of nightclub photography and the use of a flashgun is paramount! The photos will otherwise be dark or incredibly blurred, you will find taking photos of large groups with the aperture wide open causes many of the people to be out of focus and you wont have the shots you want.
With the flash gun I use it in the bounce position with a piece of white card acting as a bounce card to project more light onto the subjects giving a nice even tone, the aperture I keep at around 5.6 and the shutter speed at /25 and below going down as far as 1 second at times, really its about experimentation rather than people telling you what to do you will generate you own style.
You can see a set of my photos shot in a nightclub with hardly any light here:
http://www.facebook.com/photo.php?fb...5106997&type=3 to give you an idea of what the settings give you,
Any questions drop me a message and I will gladly assist
Jake