title: Google Cloud Messaging: How to get Registration Ids for device groups?
body: <p>I'm trying to send a downstream message to a device group, but it requires the registration_ids. I am able to get the registration tokens that are used for a regular GCM downstream send( <code>https://gcm-http.googleapis.com/gcm/send</code>) , but I'm assuming the token and id are different things.</p>
  
  <p>For example I'm looking for the registration ids here:</p>
  
  <pre><code>https://android.googleapis.com/gcm/notification
  Content-Type:application/json
  Authorization:key=API_KEY
  project_id:SENDER_ID
  
  {
     "operation": "create",
     "notification_key_name": "appUser-Chris",
     "registration_ids": ["4", "8", "15", "16", "23", "42"]
  }
  </code></pre>
  
  <p>When I send a regular downstream send message I always get a response of 0 for the canonical_id. Anyone know how I can obtain these ids?</p>