We went through it, too. Still going through it. My daughter is 10. She had bladder reflux, frequent bladder infections and eventually after 4 years, when the blood levels in her urine got so high we went for surgery. Turns out she was 9 and had the urethra of a 2 year old. No wonder she had problems. After that it was like floodgates. She was finally able to pee normally and now a nearly a year after we are still wetting the bed. So I kinda know what I'm talking about.

What I have learned:
1. Restricting fluids doesn't work. All it does is make the child nervous "I didn't drink anything, it's not going to happen tonight" and when it does the utter helplessness they feel is heartbreaking.
2. They aren't lazy. They don't snuggle into the bed and think " yeah tonight I'm gonna pee just to piss Mom off" If anything the last thought they have is "Please not tonight"
3. There is no shame. At all, ever. If you are angry at having to wash sheets every night, or even at your child for not being 'normal' fake it. Seriously.

Why it may be happening:
1.Structure problems. Maybe the growth is off a bit and the child has a tiny bladder. Maybe the valves don't close properly and the bladder fills suddenly. A simple VCUG can help figure this out.
2. There is a hormone (which I can't recall the name of) that turns on when you go to sleep slowing body functions, including the signal to fill the bladder.
3.Deep sleeper. The child may be a very deep sleeper and not notice that they have to pee, or even that they are wet and cold.
4.Habit. Your brain tells you "hey wake up, go pee" If you tell your brain "Later, man", then eventually your brain will stop sending the message. You will have to retrain your brain to send that message.

What to do:
1.Nothing. Most problems like this clear up before the child reaches 12. Right before puberty, the body grows, changes, new hormones take over and suddenly the child doesn't pee in the bed anymore.
2.Alarms. They work, if you work them. And it gives the child an active role. Some alarms hook to the underwear. Some you lay on. At the slightest wetness an alarm sounds. A VERY LOUD ALARM. It startles the child awake and they run to the bathroom, usually peeing, down the leg the first few times. After a while the brain connects the "Oh we have pee" message to the " Holy Cow what's that noise" message. and will wake up the child without the alarm. Think Pavlov's response. But you cannot use the alarm one night and not the next. And the alarm is very very loud. It's worth saying twice. LOL And the child hooks the alarm up, and turns it off, the child strips the sheets and cleans him or herself up. Every time the child wets. It completely wakes the child up, so doing this during exam time isn't a good idea, the summer months are better. It can take from 2 weeks to 3 months for it to work.
3.Medication. I'm not a doctor, so I'm not going to put my two cents in, but a pediatric urologist can give you more info.

About the sleepover: No pressure. Just pack the goodnights, like you would pack panties. Let the leaders know. Have a trash can in the bathroom. When she wakes up in the morning, have her go to the bathroom asap and clean up, changing in the bathroom. Very simple.

My daughter's best friend was awesome. She knew my daughter's "secret" and she was so very protective and sweet. She covered for Sarah when she needed to get up, distracting the other girls while Sarah went to the bathroom in the morning. maybe yo could have your daughter clue in a really good friend, maybe sit this friend down and explain everything to her. It's always good to know you have support from friends, especially away from home.

That's all I got. Give your daughter a hug for me.