I can tell you what we did from the dd's perspective.

I'm 20 right now. When I was 18, my parents gave me a trip to Disney World as my graduation present. They booked a 2-bedroom villa at the BCV and told me to invite my best friend. My best friend at the time (and still is) was 18 at the time as well. I told her first and then our parents talked out the details.
My parents obviously paid for the accommodations, as they chose the resort and it's only fair. They got the master bedroom and we were fortunate to get a dedicated two-bedroom so my best friend and I had our own queen beds.
In terms of finances, my best friend's parents treated the trip as a graduation present that they were giving her as well. They paid for the basic Magic Your Way Dining Plan (back then, $36.99/pp/day) and airfare (round trip, Southwest was around $90). They also covered park hopper tickets. There was no issue with this, as it was treated like a graduation present and her parents were happy that we invited their daughter along. My bff was responsible for paying for the items that she wanted. Just like I was responsible for paying for items that I wanted from my own spending money. If I wanted to do something and have my bff join in, we would charge it to the room key, which my parents covered. (For instance, we charged two water tubes for Stormalong Bay to the room for the whole day.)
We're all going again this January.

We booked a 1-bedroom concierge level in AKL's Jambo House. Parents are again in the master bedroom and my bff and I are going to sleep on the pull-out couch or maybe one of us will sleep in the sleeper chair if it's comfortable. Again, my parents are paying for the accommodations. This time around, since there's really no special occasion, my parents are covering all meals that we eat together with them. If my bff and I go on our own and decide to buy a snack or counter service meal, we're on our own. My parents are really lax though and aren't picky if we charge things to the room card. I think my parents would cover the meal plan if we actually decided to buy it, but we're staying concierge level and plan on eating breakfasts and snacks throughout the day. My bff this time is responsible for airfare, park tix, and spending money, as last time.
I don't think it's rude to invite another friend and having them cover their own expenses. Of course, I'm 20 and reliant on my parents, but when my friends' parents invite me on a trip, my parents pay my portion. If my parents were not OK with that, I wouldn't be allowed on the trip- enough said. It really depends on where you're going and what you're doing. Everyone knows that Disney is expensive and it's nice to have part of your trip covered, but I would never expect other people to pay for everything for me.
This is quite common, actually. I went to a prep school (boarding) and it was common for students to go home with their friends for the holidays. If transportation was via plane, the student would pay for his/her own airfare. Of course, room and board usually was covered by the parents, but the student usually paid for what he/she bought/went to with their friends if the parents didn't cover it. That was assumed, and quite the norm. Who knows what's it's like now- I graduated three years ago- but I always consider these types of trips treats in the way that I'm happy to be invited.
Have fun! It'll be a blast- and as parents, it'll be nice to get your daughter/son out of your hair.
