Ask for the route and see if you can drive it beforehand.

Personally, I'd be a bit nervous riding with a group I've never ridden with before on an unknown route in the dark, but I'm a huge chicken. Having said that, the shop I bought my bike from hosts a night ride on one of the local trails, which sounds like fun...but that's flat.

And just make sure you don't blind the people behind you! I second the glow stick suggestion--bright, but not "My retinas!"-bright