title: GCM with multiple senders
body: <p>GCM introduces multiple sender functionality, which allows multiple users to send push messages to the same application. Multiple users are given the same registration id.</p>
  
  <p>If the number of multiple users is three - Andy, Bob and Chris - and Andy is blocked by Google from using the GCM service, is the received registration id still valid?</p>
  
  <p>If the received registration id is blocked by Google, are the others (Bog and Chris) also blocked from sending push messages?</p>
  
  <p>How does Google manage and block malicious users?</p>
  
  <p>In addition, I'd like to know how does Google manage registration id.</p>