It all depends on the light in the club. How much? What type(s)? how directional is it? The answers to these questions determine what approach I take.
In general I choose to shoot with one of the two following approaches:
- Flash: A fast normal zoom. Something like a 24-70 f/2.8 (17-55 f/2.8 for APS_C camera). A small E-TTL flash either mounted on the hot shoe and bounced off the ceiling/wall or handheld in my left hand about 2 feet or so to the left of camera and aimed directly at the subjects. Sometimes I use a hot shoe cable (easier setup), sometimes a wireless transmitter/trigger (more flexible). Gel or use an orange or green Sto Fen omnibounce or similar cover on the flash to match the predominating color of the ambient light and try to use as little power as needed to allow the ambient light to influence the mood of the shot as well.
- No flash: A fast prime lens such as a 50mm f/1.4 (35mm f/1.4 for APS-C). Crank the ISO up and worry about the noise in post. Don't be afraid to go up to ISO 3200 or even higher. Use the widest aperture you can that allows the depth of field you need. Try to compose group shots where everyone is the same distance from the camera.
The first approach. Canon 5D Mark II, EF 24-105mm f/4 L at 35mm, ISO 800, f/4.5, 1/60 second, 480EX II in E-TTL mode w/-2/3 stop flash exposure compensation.
And the second approach. Canon 5D Mark II, EF 50mm f/1.4, ISO 5000, f/2, 1/50 second. Notice the lack of hard shadows. It wasn't nearly as bright as it looks in the photo. If I hadn't have been primarily shooting an act on the stage (and thus wanted to avoid flash and not even set up with one on the camera) I would have chosen the other approach to this shot. At this display size you can just begin to see a little bit of camera movement. The short duration of a flash would have frozen the subject and allowed a faster shutter speed.
Just because you use a flash doesn't mean you can't catch the mood of the ambient lighting. You do need to carefully balance the two by using only enough flash to light your subjects without killing the ambient. Canon EOS 5D Mark II, EF 24-70mm f/2.8 L at 32mm, ISO 5000, f/4, 1/60 second.
Canon EOS 5D Mark II, EF 24-70mm f/2.8 L at 24mm, ISO 5000, f/4.5, 1/60 second
Note: The first two photos were shot in the same club on different dates under fairly similar lighting conditions other than the first was not near the coolers behind the bar lit with fluorescent lights. As you can see, using flash sometimes allows you to preserve the mood better because you can let the ambient light in the background stay dim without leaving your subject too dark. But you have to really be careful not to use too much flash power!
In either case be sure to save your files in raw format so that you will have control over the color when editing. In the second shot above there were very dim incandescent lights overhead, fluorescent lights in coolers behind the bar to the subject's left (seen reflected in the framed art), LED lights illuminating the stage to camera left (the magenta reflections on the subject's right sleeve, side of his face, and some of the bricks), and beyond the stage were large view windows looking out on a street lit with sodium vapor lighting. There would have been no way to get anything approaching decent color without being able to work with the raw data in post.
Addressing some of the specifics of your question:
- The kit lens you have is too slow to use in a dark club without using any flash. So is just about any zoom lens, even a constant f/2.8 zoom. Some clubs are just bright enough to get away with f/2.8, but you will need to go way past ISO 800 to do so. You're going to have to learn how to do noise reduction in post.
Unless all of your photos will be taken at the same distance, you really need to be able to control the flash via E-TTL and Flash Exposure Compensation. Otherwise you'll miss most of your shots trying to adjust the flash power while the moment disappears.
If you are renting lenses go for fast primes. They're cheaper and better suited for a dark club. But you need to be able to visualize what field of view they give you and how much room you'll need to frame one person, two people, three people, etc.
You probably should have never accepted a paying assignment until you've shot the same kind of situation on your own dime first. No matter how much you read and get from others, you're going to discover a lot of how to shoot in such a challenging environment the hard way: by learning from your mistakes. Many of those won't be obvious until you are sitting in front of a monitor looking at them. The LCD screens on the back of cameras lie like politicians! If possible, get to the club a few days before the gig and practice! Then look at the results critically on a good monitor.