Ambush,
I hope you know your friend real well, along with his friends that he's invited.
Going on the track in with a bunch of guys you don't know has it's own set of problems; racers looking for track time, complete newbies, etc.
If one guy dumps it, you sit on the sidelines while the one ambulance drives 1 hour away, and you wait about another hour before another ambulance gets to the track.
I've done this before (organized years ago with some VFR guys), and it is a blast. I would simply be wary of the others riding on the track with you.
Hopefully, you have the "full course" open to you... Thunderhill is a lot of fun! Just don't **** around on the Cyclone; it ain't worth it!