Sum with a group by Author
Not sure if this is the best way to do it, but I created this little function in a Variable class.  I get all the necessary info (username and score) in an array.  Works fine for what I needed.

public function leaderboard(ElementCriteriaModel $criteria)
   {
       $query = craft()-&gt;elements-&gt;buildElementsQuery($criteria);

   if ($query)
   {
      $query
          -&gt;select("authorId, SUM(field_score) as score, users.username as username")
          -&gt;join("users", "users.id = authorId" )
          -&gt;group("authorId")
          -&gt;order("score DESC");

               return $query-&gt;queryAll();
       }
   }