Settings to use for group shots at night?

I have a nikon d90 with 35mm 1.8G lens and an sb 800 flash. When i take group portraits what settings do i use for my camera and flash to only allow the flash to light up the people in the picture and not the background?

I guess you asked your question in the wrong category. You may try: Games & Recreation: Hobbies & Crafts OR Other - Games & Recreation. Or maybe you'll find a suitable category in Consumer Electronics.

The known:
Your subjects need to be at the same distance from the camera.
Use a tripod.

The unknown:
The amount of people in the portrait.
Ambient light in the area.
Type of ambient light in the area.

Every photograph taken have their own distinct settings. It is a fine mix of distance, lighting, aperture, white balance, ISO, and composition. The SB800 can be adjusted to produce a verity of intensity of light, you can start at a high output and work your way down to get a more balanced light. You should try and use a diffuser of some type to minimize the sharpness of direct light.

Good luck, Post more information you may get a better directed answer.