First, I would suggest that you instate a rule that means the kids clean up every 1 or 2 hours so things are less daunting at the end of when it is time to leave. Another way to meet the same goal is to clean up each activity that gets done before moving on to the next activity. There should never be a situation where more than one room is messy at a time or you are setting yourself up for failure in the end.
Second, During a sleep over, I have the guest children pack up their things in the morning as a part of "getting ready to start the day." Pajamas, toothbrush, hairbrush etc. All go in the overnight bags right after use and I help with rolling up or putting away sleeping bags before they even get to start playing. This helps to make sure there are only a couple of things to check for when it is time to leave and allows the parents more ability to focus on directing their kids to help with clean -up.
Third, about 30 minutes before it is time to leave announce, "Okay everyone! Clean up time." I commonly say something like, "Who can get 15 toys picked up first?!?" Then I make a really big deal about how fast they are being and who is in the lead. I try to sound like the announcer for a race and really play it up.
If the kids don't help and the parents are there but aren't helping to teach the guest children the idea that they should help with clean up time, there isn't a lot more you can do without having a chat with the parents of the children. When the guest children's parents are in on it with you, they can take over with motivations they regularly use at home if "making a game of it" doesn't help. When, how, and if, you decide to do that will depend somewhat on your relationship with the parents.
Hope it works for you - have fun!