just for other's info, i e-mailed redline events as steph asked, and here's the answer:
"Here are the answers to your questions:
"Passengers are allowed in cars after the morning sessions. You should only
have one passenger with you while driving on track. All passengers are
required to wear a helmet, but there are no fees for passengers.
"There is no extra fee for other drivers to share the same car. Your track
fee is for one spot at the track. If you wish to let other people use your time
on track, that is up to you and it doesn't cost extra. If you have another
driver who wishes to share your car, but wants to run in a different run group,
then a second track fee is required."