This game works, but not too many people can take part in it.

This game requires an account on a site where you can post videos (such as Youtube) and any game, but I suggest a Mario Party minigame. You post a gameplay video on that site and we have to guess which character you are. Although I have a Youtube account (WiiParty2010), I don't have anything to take videos of, so I am unable to participate in video making. Multiplayer (online or not) can be used as well.

I also advise to pick a game that does not automatically reveal who you are (such as Mario Kart, Wii Party or Mario Party 9).

P.S. Free Play can be used, just don't show the Free Play minigame selection screen, or minigame instruction screen and don't mention that you used Free Play.