How do independent components communicate in ember?

How do I let independent component let know of changes or events in a component?

eg:

 <#user-social profile>
  {{partial 'user-handle'}}

  <div class='subtext'> 
    {{#reply-component}} {{/reply-component}}
  </div>

  <div class='replybox hide'>
    <textarea></textarea>
    <input type='button' value='Reply to user' />
  </div>
 </user-social profile>

Problem: I want replybox to toggle its visibility when a link inside reply component is clicked.