In my own opinion, your daughter should already know about sex, hygiene, drugs, theft and current events. Yes, I understand that you might temper the information for her age, but she needs all the information she can get from you -- especially before she matures enough to go through the, "I'm not listening to you." phase.
You help her learn how to stand up for herself and against peer pressure. I find that there are usually a few movies or TV shows that I can watch with my child that starts the conversation in a non-threatening way. "What did you think when she stole the fruit?" "Why do you think she agreed to do something she knew was wrong?" "What could she have done instead?" You teach, you listen, you guide her to good ideas, and you show by example how you expect your child to react under similar circumstances.
So in this example, I'd call the parents and ask what the plan is. Will there be older girls or boys in the home? Will there be a bedtime? Does anyone smoke? Are they watching TV or a movie? Is it a movie that is rated appropriately? If you have other questions, write them down and ask. If you are concerned that the parents might drink alcohol and you need to know -- ask. Yes, it is uncomfortable but this is your child -- and even in another home, she is your responsibility.
I think the idea of hosting your own slumber parties is good. No one can say you must send your child to another party, but if you host some, your child will have the experience in a way that you are comfortable with.
I loved slumber parties. We stayed up all night as as we got older, we snuck out. It was part of making mistakes -- but sort of safe ones as we were in a group. I tried my first pot at a slumber party. My point is, kids are going to do what kids do. I did not need a party to smoke pot -- it was everywhere and if I had not had it in the relative safety of my friend's home, I'd have tried it at the park or mall.
Preparation is the key. Tell the truth about your fears -- but be accurate. Know that now is the prime time for teaching your child how to be in the world. Very soon -- sooner than you would ever think, she'll be doing things that you know nothing about. By the way, I was eleven when I started smoking cigarettes and pot -- and my parents did not know until I was around fourteen.
If your daughter is not the sort of personality to say, "No." and stick with it under pressure, then you do need to be very particular about where she spends the night.
She needs to know that no matter where she is and no matter what the time is or if she has broken a rule or outright disobeyed you -- that one call will get you there as soon as possible -- no questions asked. That doesn't mean there will not be consequences, but that no matter how wrong she is, you are her parents and you'll come.