I've done quite a bit of shooting in clubs, which really came in helpful when the first wedding I shot had their reception at a tango club in the middle of the night. All clubs are going to be different and require different things, depending on what you're trying to accomplish. For individuals, you just need a good flash with a zoom option, and possibly a small softbox or snoot to control the light:
Exposure: 0.006 sec (1/160)
Aperture: f/8.0
Focal Length: 35 mm
Focal Length: 35.0 mm
ISO Speed: 800
For action shots, the flash is also critical, but it helps to get practice prefocusing, so a fast lens is STILL helpful to give you the most light when prefocusing.
Exposure: 0.017 sec (1/60)
Aperture: f/3.5
Focal Length: 35 mm
ISO Speed: 100
For group shots, what works the best is going to vary. Sometimes it's more helpful to have a faster lens and work with available light, and other times you're stuck with a flash. Often, clubs have dark ceilings and bouncing doesn't work particularly well, but that's often going to get the best result when it's available.
Exposure: 0.006 sec (1/160)
Aperture: f/3.2
Focal Length: 60 mm
Focal Length: 60.0 mm
ISO Speed: 200
Not that I'm trying to claim these are the best possible results, they just happen to be some examples I have handy of doing different things with different methods.