title: Centos usermod -a don't add a user to a group
body: <p>In my project i have to create a group on my centos server and add two users; i do:</p>
  
  <pre><code>sudo groupadd editorial
  </code></pre>
  
  <p>then i try to add my two users to group like this:</p>
  
  <pre><code>sudo usermod -a -G editorial nginx
  sudo usermod -a -G editorial ec2-user
  </code></pre>
  
  <p>ok, no errors, but when i check my group:</p>
  
  <pre><code>sudo groups editorial
  </code></pre>
  
  <p>in output i get:</p>
  
  <blockquote>
    <p>groups: editorial: no such user</p>
  </blockquote>
  
  <p>why i cannot add my user to group?</p>
  
  <p>Thanks in advance</p>