(: XQuery comments are marked by mirrored smilie faces, like this. :)

An empty query is not valid. There needs to be something besides the comment

  <users>
  {
    for $user in //users
    let $comments = //comment[@uid = $user/@id]
    where count($comments) > 0
    order by $user/lastName, $user/firstName
    return <user id="{ $user/@id }">
      <name>{ concat($user.firstName, " ", $user.lastName) }</name>
      <comments count="count($comments)">
      {
        for $c in $comments return <comment id="{ $c/@id }" />
      }
      </comments>
    </user>
  }
  </users>

http://www.stylusstudio.com/xquery-flwor.html