Shot some photos in a disco last year, used settings that Tdodd recommended..
Camera: Manual, 1/60, F8 and ISO 800-1600
Flash: TTL -1/3
The flash froze the action and the high ISO took in the ambient light.
I can't believe ever recommending f/8 for something like that. For a wedding reception type shot then 1/60-1/100, f/2.8 and 1600 ISO is my typical starting point. Bounced flash takes care of the shortfall.
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But the exact settings do need adjusting to take into account the actual ambient light levels and the desired visual effect. Do you want shallow DOF or not? Do you want the background dark or properly exposed?
One thing is for sure - I'd set the background exposure manually, so it wasn't fracking about all over the place as the guests arrived wearing all manner of attire.
Practice beforehand would be a wise precaution, if you are able, unless you have the confidence to know what adjustments to make on the fly.
For failsafe, shoot raw and switch to P (for Panic) mode with evaluative metering, and hope. Check your histogram and look for clipped highlights in the preview image. If you want to adjust the brightness of the background you can use the EC dial. If you want to adjust the brightness of the subject(s) then use the FEC control. Do not use Av mode under any circumstances. Try to position yourself so that there is nothing in the frame that will reflect flash light straight back at the camera, such as windows, mirrors, large expanses of white wall or anything else that could screw with the metering.
Here's an example with my 30D with a manual exposure at 1/60, f/2.8, 1600 ISO and bounced flash at +2/3, no edits except white balance and some sharpening. If this is close to the look you want then at least you have a starting point from which to work....
You might also want to think about focal length, subject distance and perspective. Will you be fighting for elbow room and need to get in close with a wide angle, or will you have a clear shot from a little further back and be able to narrow the FOV and exclude a lot of the background? I don't need the answer, but you should give it some thought.
Good luck.
