I am sorry to hear that you are experiencing problem with the App. Please let me know the maker and model of the device you are using. Also, let me know if in game you appear as part of the Clan.
We are aware that some players are experiencing functionality issues with the app at the moment and Beachhead is currently working on it. You can find more info/updates here Twitter / beachheadstudio: We're aware of some intermittent. In the mean time, ask the clan leader if they can invite you. I would also try relinking your game account to your Call of Duty account as well as re-logging into the app.