(need help/advice Halloween costume party)

There is a charitable event in town on saturday night and I'm volunteering to take some pictures (there will be other photographers there too)

I thought this would be a good chance for me to get some experience.

Up to now I have mainly done sceneary.

The kind of shots I want to take it people mingling on the floor...the art around them and any speakers that come up.

Now I don't expect the lighting to be that good as it is a gallery, so I'm going to be bring my flash but I'm a newbie flash user..I figure if I point it upwards I want get those shiny spots or eyes in my pictures.

Should I use my sigma 17-70 as my main lens and bring my 50 mm?

This is a small venue of about 200 people and the gallery isn;t too big so it will be tight.

Any help /advice/ tricks /tips would be very very appreciated

My limited gear is listed below I also have the kit lens

Do an internet search on "A Better Bounce Card". I've tried it and find it better than a Stoffen. I hate the way a Lightsphere looks. Both cut too much of your flash power. If the gallery ceiling is too tall or nonreflective, you won't get adequate bounce to light your subject. I used glossy photo paper and a thick rubber band (to secure the paper to the flash head). Bounce card helps direct diffuse light toward the subject without the harsh effect a direct flash has. Works great, really inexpensive, simple, and you can make 1 small card out of 4X6 paper and another larger one when you need more light reflected and just keep them in your camera bag.