This well-known game is great for language learning, for modeling participatory learning, and for stimulating thought about a theme.
The flow is easy: the joker stands at the center of a circle of seated people facing in. The joker says, "I have a letter for everyone who..." and adds some description. For example, "I have a letter for everyone who has glasses." The people with glasses then have to move to different chairs. The person who ends up without a chair becomes the new joker.
The game can be made thematic by using cues that are related to a theme: "I have a letter for everyone who is against nuclear power."
To keep the game on a theme, the joker can also prepare a list of cues ahead of time, and each new joker can choose an item from that list. In this way, the game can also be a kind of diagnostic activity, finding out what features or ideas people share.
You may want to designate an observer or two to take notes on which items are shared by everyone and which are shared by only a few.