(Photo from Canadian Film Centre)
This game can be played with as little as four or five people, and is fun for all ages! This is a game that I used to play with my kids when I taught acting and improvisation classes.
The basic premise is that one person is hosting a party, and several guests come in as various people or personas. The host must talk to them and casually say who they are.
First, the host must leave the room (or put on earmuffs, just overall be unable to hear what everyone else is saying as they organize the game). It's best to pick the host at random and alternate between different kids so they all get the chance.
While the host is distracted or not listening, all of the other participants ("party guests") have to decide who they are. People can be famous real people (Justin Bieber, Selena Gomez, Abraham Lincoln, etc.), fictional characters (Hermione Granger, Hannah Montanah, etc.), or even vague descriptions (such as a banker, a supermodel, a pop star).
Once everyone is settled on a character, they leave the room (or go to a corner of the room out of the way) and wait by the door (real or not). The host comes in, and one by one, the party guests knock on the door (usually best to be about one minute apart).
As each guest comes in, the host welcomes them and has a conversation, trying to casually learn more about the person without acting like they're a total stranger our outright asking them obvious questions. The guests take on the entire persona of whoever they are (accent, mannerisms, walk, etc.), and the host should try to guess who they are based on this and conversation.
It's usually best if the host tries to slip their guess into conversation instead of saying outright "Are you Harry Potter?" so as to make the game conversational and long-lasting; rather, the host should say "So when do you go back to Hogwarts, Harry?" or something similar.
This game works great for kids 7-18 and really is a lot of fun once they get going; they're all going to be dying to host the next party! Feel free to contact me if you'd like more ideas; I have TONS I've played with my acting kids!!