I have the Canon 10-22 and use it sparingly for group shots. The times I usually use it, is when i am taking a wide shot of the entire club or crowd, then I see a group of people I want to take a shot of, or someone asks me to take their pic and not enough time to change lenses. It is a great lens, but I don't think it is compatible with the 300D. It also freaks people out on how close i am standing to them while taking the picture. With it at 10-15 mm, I would be like 3-4 feet away from a group of people, which is really odd to them.
As for the flash, I only use the built in wide panel, and catch light card when I have it in bounce flash. I also use full manaul and dial down to 1/32/ - 1/128 . Its all about trial and error.
Every club/ bar/ concert lighting is so different.
Another thing I have been playing with is turn up the color temp to give evryone a nice tan. Normal setting is arounf 5200, but if I bump it up to 7500-9000, it gives everyone a nice tan. I honeslty alomost throw out the stuff like normal skin tones, correct exposure etc, in clubs.
