title: How do I get the email address of a Firebase user by ID
body: <p>I'm using email/password authentication in Firebase. So every user has an email address. My users are in groups, and I want each user to be able to see all the other users in the group. Specifically, I want to display a list of emails in the frontend to the user. My question is not about how to make rules for that, but rather, <strong>how do I get a user's email addresses given a user ID</strong>? Note that I'm asking about getting a list of <em>other</em> users, not the currently signed in user.</p>
  
  <p>I haven't found any SO answers showing how to get the email address of another user via <code>auth()</code>. I've seen suggestions to place the email in the <code>/users</code> collection under the user ID, but that seems incredible brittle to me to store the email addresses in both <code>auth()</code> and <code>/users/$userId</code>. Changing email addresses will be a nightmare. So there must be a way to get the emails from <code>auth()</code>, right?</p>
  
  <p>Thanks!</p>