If your daughter wants to have sex with a boy, she will do so and trust me, you won't know anything about it unlesss there is an urgency to tell. The fact that she tells you the truth about the situation speaks to the trusting relationship you and she have. If you know "the boys" who would be part of this, and you have no ill feelings toward them any other time, why should a sleepover be different.
Now that said, I am sure that she will also understand that you are not entirely comfortable with the plan. Good luck with that - and again -----soooo glad I ain't got kids.