If you're doing promo shots for the nightclub in question, then I assume you're more interested in capturing the atmosphere of the place rather than the identity of the patrons. If this is the case then I would actually recommend against using a flash at all, or at least limit it to a very weak bounced or diffused flash.
Flash will certainly make it easier to photograph people, but it can totally kill the ambience created by the lighting inside the club.
Instead, you'll need a fast wide lens, a camera that performs well at high ISO, a steady hand (and/or image stabilisation and/or a monopod), and hopefully a crowd of people who aren't moving around too vigorously. And take LOTS of shots, there will be plenty of duds. Shoot in RAW too so that you can easily adjust the white balance on the PC later.
Also get used to shooting in manual mode and possibly manual focus too. The camera will most likely struggle to give consistant exposures as it gets confused by a dark room with moving bright lights. It's often best to find a aperture/shutter speed/iso combo that works and then use those settings in manual mode. Similarly the camera can struggle to lock focus in low light, and if you're focusing through a moving dance floor then you may be better off focussing manually.
Here's a shot I took last night of some swing dancing at The Mustang Bar
here in Perth. I used a Sigma 28mm f/1.8 lens (using manual focus). I also had the 30D in manual mode, shooting at 1600 ISO, f/2.2 and 1/100th sec.
Here's another shot I took using a slower lens (Sigma 10-20mm f/4.5-5.6 @ 20mm). It's still 1600 ISO, but this time f/5.6 and 1/13th sec. You can see how much motion blur occurs due to the slower shutter speed in this shot. This can be a good or bad thing depending on the circumstances and what you're trying to achieve, but either way it is something to be aware of.
This final overview shot of the crowd illustrates another reason why its good not to use flash in a situation like this. If I had used a flash for this shot it would illuminate the people in the foreground and the camera would expose for those people, making everything in the background become darker as the light from the flash falls off. If you want even lighting over a large room then you need to use the ambient light instead of a flash.
This shot was with an Arsat 35mm f/2.8 tilt/shift lens, 1600 ISO, f/2.8, 1/25th sec. Note that even though this used a fairly slow shutter speed, the motion blur isn't as obvious because the people in focus are a fair distance from the camera making their gestures appear smaller in the image.
I hope that helps a bit. Its not an easy environment to shoot in. I suggest you visit one night to do a test run first to suss out your settings, as the lighting in your club will definately be different to the Mustang Bar.