shooting prom pics tonight - advice?

I've got 2 family members that want me to shoot their prom pics tonight. They will be within an hour of sunset i think. But say if they were taken at night, one will but under a stone gazebo with some not-so-bright lights on the ceiling.

How would i want to shoot this with a flash? Av mode? M and drag the shutter to expose background?

I'm shooting my first pre-Prom tonight too! I was hoping to do more learning than informing but here's my best shot.

I'll probably shoot manual and bring a portable PC to take a look at the exposure. I plan to use a 24-70 f/2.8 and will probably stick at the 2.8. I bought a collapsible reflector to try and brighten the faces. I've got a couple of flashes, umbrellas and pocketwizards in case I need some additional light.

I'll continue to monitor this thread so hopefully some more experienced folks will contribute.

well i just got a 580exII yesterday and havent even had a chance to use it so i guess i'll stick with ETTL

anyone? ETTL? Av, M, or P?

bump...