title: Common Membership Algorithm
body: <p>People can belong to one or many groups. What is a good algorithm to output common memberships? </p>
  
  <p>ie, Persons A and B are in Groups C, D, and E ... etc</p>
  
  <p>My preferred language would be Ruby (or <em>maybe</em> Python), but any code or pseudocode would be greatly appreciated.</p>