My daughter is 19 so I know that this isn't an easy decision. First of all you should be thankful that your daughter would tell you in advance about the sleep-over. She sounds like a very responsible young woman and you two must have a good relationship or she wouldn't have let you know. You can't babysit a 17 year old 24 hours a day. There has to be trust and open communication. Talk to your daughter some more, call the parents and talk to them too. Offering to help supervise is a great idea. Try to work out a compromise that everyone can live with. It may involve an early morning brunch (think 2 a.m.) and you driving some sleepy teens home at 4 a.m. Just telling your daughter no may work this time but the next time she just won`t tell you. Good luck with all of this and keep talking.