title: how to indentify user using google apps script?
body: <p>I need a way to identify users. I have a Google site that is users access via their gmail accounts. My site is designed to allow users to view any data, but edit only their own data. This works fine for me as I'm the owner of the site and publisher of the scripts, but <code>Session.getActiveUser().getUserLoginId()</code> returns <code>null</code> for anyone else.<br>
  Please provide me the solution about this problem. Any link / any piece of code?</p>