If you haven't had experience with off camera flash I wouldn't try it for the first time at the event.
Your problem will be ugly overhead lighting, and flash recycle times.
I'd tend to use flash on camera, but only under expose ambient by 1 to 1.5 stops so the flash is more subtle, and recycle times aren't as bad (put camera on manual; set shutter to 1/180, ISO on ~400, and see what aperture you need to underexposre a stop of so, you might need to increase ISO if the light is a bit dim). If you have good light then maybe put a stofen on the flash head, it eats a stop or 2 though so could hurt recycle times too much. Then dial the FEC down on the flash to about -1 or -1.5. Take a test shot and see what it looks like.
Ideally you'd take a shot as they walk towards you, and that gives the flash enough time to take a second shot as they pose in front of you.
Good luck