Definately call the parents and ask if you can visit for a few minutes to talk. Then visit and get to know them - ask lots of questions.
See which parent is staying up all night to keep an eye on them.
(How about offering to go there for a few hours (say 1am-5am) so they can get some rest before breakfast?
Call the other parents involved...maybe all the kids are saying "all the other parents are letting their kids stay" when they may be feeling just like you.
How well do you know your daughter? Do you know the friends? What about their parents? What are they like? Have they been over? Are they respectful? What are they involved in at school? Good students? Bad students?
I'm sure there's lots of terrible, horrible stories out there but there are good stories too you may have never heard of either. I think it's great for girls to have friends who are boys and boys to have girls that are friends.
The *KEY HERE IS TO KNOW YOUR KID, THEIR FRIENDS AND THEIR FRIENDS' PARENTS*. It's starts in grammar school and should continue throughtout high school!
Then make your decision...good luck...
By the way - been there done that.