Regarding for your question, there are two different answers for it depending if which game you are asking about.
If it's for Ghosts, you need to download the Call of Duty app for free on your mobile device's respective OS' app store. Like iTunes for Apple and Google Play for Android and the App store for Windows.
If it's for older titles like BO2 or MW3, I'm afraid to say that Elite has already shutdown and along with them, is the option for all clan functionality
on the game. Here is a link for more details: http://support.activision.com/articles/en_US/FAQ/Call-of-Duty-Elite-Shutdown-FAQ /