title: Stop Unauthenticated Cognito Identity User Pool Signup
body: <p>I've been playing around with the new Cognito User Pools to manage user accounts and I'm trying to build a system where users have to log in to use the system, and new users can only be added by existing users (ie no public signup).</p>
  
  <p>Currently there seems to be no way of stopping an unauthenticated user from calling signUp.  I've changed it so that the Pool doesn't allow unauthenticated users but this doesn't seem to make any difference (as setting the accessKeyId and secretAccessKey to anything makes it run).</p>
  
  <p>Have I missed something?</p>