title: How do you create a unique identifier for users for tracking purposes?
body: <p>I would like to be able to add a user referal params on all invite links sent out from my site... example:</p>
  
  <p><a href="http://site.com/invited_by?=ajraux" rel="nofollow">http://site.com/invited_by?=ajraux</a></p>
  
  <p>How can I generate a code that is short like "ajraux" for all users on my site? Does it need to be a field in the database? Can it be created on the spot? How are web companies doing this?</p>
  
  <p>Thanks</p>