In my test (two accounts, two devices), all 4 of my girls stayed the whole time. And while they were at the party, they were also working and hobbying elsewhere. My girls were the only guests, though. I understood maybe they cycle in and out if the party has a whole lot of guests?
When I have my party, if you make it, I'll be more than happy to keep track. I think it would be extremely helpful to know. More experimentation! 😄 Sims Science!