Going to hop down on the OTHER side of the fence on this one... I tend to be a parent who encourages independance in my kiddos, and I am very liberal in my thinking, so take the following with a grain of salt.
As several PPs have posted, in another year or so 17 yr olds will be off to college, and at that point you will have absolutely NO SAY in where they go and with whom. You have to hope at that point that you have given them the morals, intelligence, and strength to make the right decisions.
I can remember being 17-ish, making plans for a random Saturday night, and always having that one friend who always had to run EVERYTHING by her parents, who always had to be in by 11, who was never allowed to do ANYTHING... I always felt bad for her, and when I saw her around town a few years later it was painfully obvious that as soon as she was out from under her parents' wings she went violently in the opposite direction and had had a really ROUGH couple of years. I generally feel that, about the age of 17, parents have to gradually start letting go. I want my children to have the freedom to fail while I can still help them get back up again.
I think I would call the parents of the other kids... make sure they were aware of the arrangements being made. I would ask them how long their son had been driving, and what his record was. i would make sure the car could hold that many kids. If it all checked out, I think I would let her go.
NOW. If it were illegal in my state for a 19-20 yr old to be driving with other teens in the car, then sorry, that's a no-go. Must obey the laws, kiddos. I know in MT, for the first 6 mos you have your driver's license you may only have one teen in the car with you, then for the next year, no more than 2 teens. After that, you can play tour bus driver if you can fit 'em in your car.
Ok, all done writing a book now.
ETA: Sorry, Kelly - I see we posted at the same time. I am glad you made the decision you feel comfortable with.