I have to say I find a question like this to be how can I say odd.
You have moved up from a 40D to a 5D II
May I ask why you got a 5D II?
? why's it odd Chaz? Guy obviously doesn't do a lot of flash stuff, for all we know he's a landscape chap and spends his life shooting them, hence a little help from more portrait orientated people is handy when something like this comes up. Also poss more importantly he asked how to use a speedlight at a party, not why he made an upgrade
The approach to this massively depends on the disco tbh.
For clubs with funky light shows I like to use fairly long exposures (manual camera setting) down to like 1/10 of a second, used f2.8 on my 50d but you might need f4 or above on ff for the depth of field, then pick an iso value like say 800? That should give funky lights but not really expose anything, then use the flash on ttl to light the subject, stofen bounced off a ceiling if ya got it, not if you haven't
For more grown up parties with more light I like to expose either for a nice burn around the lights or just the room down a stop or two (another this is to expose for outside the windows if its daytime and they're likely to be in frame). Then make up the difference with ttl flash again bounced if you can
Those are both rough guides as I don't know the exact situation you're going into.
Remember to work the exposure compensation on the back of the flash to suit the subjects (it wants to meter the scene to be grey) ie -ve compensation for someone wearing black, +ve for someone wearing white
Dunno how much of that was teaching you how to suck eggs but hopefully a fair bit is useful
