Instructions
Decide on a clan name. The name can be a maximum of four characters with various symbols, letters and numbers. Infinity Ward, the "Call of Duty" developer, has restricted the use of certain profane clan names.
Boot up one of the "Call of Duty" games and go to the multiplayer lobby. Select "Barracks" and then "Edit Clan Tag" to bring up an onscreen keyboard. Enter the desired name of your clan and press "Start" to add it as a prefix to your standard gamer name.
Have your friends do the same. There is no limit to how many people can join a clan. Just have them enter the same clan tag as you. You can also recruit other players via the Internet to grow your clan.