here's somethign you can try. I assume you have a Digital SLR.
Go into manual settings, set a small aperture and a long (2-5 second) shutter speed. But here's the kicker, still use the on camera flash, and itll define people just enough and still give you lots of movement.
If you need an idea of what it will look like, go to
www.skmartist.com and look at their party pics. I shot them all, and yes half are just people hanging out, but usually the last third of them are much more arty and you can get an idea.
Good Luck!