title: Get the number of users subscribed to a pusher channel
body: <p>I'm implementing an application using vue.js and Pusher (for realtime multi-user communication). The application provide that each user authenticate himself (accessing the home page) and ,before starting, he sets some settings in the home page (about his role in the application). The subscription to the pusher shared channel takes place when accessing the home page component. I need each user to be unable to select anything in the home page until all the other users subscribe to the channel (accessing the home page), that is the settings section in the home page has to be enabled for all the users only if everyone is subscribed to the channel. I'm a begginner with Pusher and in the pusher documentation I found that there is a way to get the number of the user subscribed to the channel(on which to perform a control to achive my purpose) using the property "subscription_count" after enabling this possibility in the "App settings" section of the pusher dashboard.
  In particular this is what I found: <a href="https://pusher.com/docs/channels/library_auth_reference/rest-api" rel="nofollow noreferrer">https://pusher.com/docs/channels/library_auth_reference/rest-api</a> (in the "GET channel (fetch info for one channel)" paragraph). However, I don't understard if I can get the subscription preperty directly through the channel instance with this syntax: <em>pusher.info.subscription_count</em>, where "pusher" is:</p>
  
  <pre><code> const pusher = new Pusher('********', {
      cluster: 'eu',
      encrypted: true,
      authEndpoint: 'https://*****/pusher/auth'
    })
  </code></pre>
  
  <p>or if I have to use another syntax.
  Moreover, I want to know if this property updates dinamically in order to enable the setting section (for example using a v-if condition based on the subscriptions number) only if the count is equal to a specific value. I hope that you can help me.</p>