Good thread. Excellent advice from Kiwi in particular.
No reason why you should need more than
ISO 400, maybe 800 if the room is reasonably small and you are using a 580EX II. (Any room you can comfortably fit 20-odd people in should be fine). That's getting fairly low
shutter speeds and requiring a moderate
aperture, but nothing you can't handle with care and attention. You will probably find that you don't need much FEC, possibly none at all.
Always try to avoid aiming the flash straight at the subject, it only really works if you have off-camera flash (such as a bracket).
Bounce off the walls if you can, ceiling if you can't, diffuser if all else fails.