Shooting a gathering, I presume like a BBQ party or birthday party or something like that.
First, shooting at night with flash the exposure comes in 2 parts:
- ambient exposure
- flash exposure
Ambient exposure
If you rely on the camera P or Auto mode you are likely to get horribly black background with no hint or where or what is going on. To get some ambient, one typically set a higher ISO, like 400 or 800 (assuming you are using a current crop of cameras like D80, D90, D60, D300 etc, not D70, D40, in which case ISO800 may be too noisy to be used). Then use 1/60s or so at about f/4 - even P will do, although I normally go M completely. See your LCD, it should be between -1 to -4EV, under exposed but not too under to get some ambient recorded.
Flash exposure
You can rely on auto flash to do the flash exposure, with a caveat - it may over expose your main subject. Dial in a slight minus (-0.3 to -0.7EV) on flash exposure compensation if this happens. Or set stronger under exposure for flash if necessary, if too dark, set it for over exposure. The flash exposure compensation is near the flash release button, press it and you will see on the LCD a bolt with +/- beside it.
Try it out.