How to create a hierarchy of users

Ok, I've managed to sort some of this out myself. The following foreach loop:

foreach($users as $var => $value)
      {
      $profile = $modx->getObject('modUserProfile', $var);
      $v = $profile->get('fullname');
      } 

successfully displays the last member of the group. What I need to do is:

• return an array of all group members.
• somehow get this array into a select list (preferably one that looks and acts like the Gender select on the new user page).

I have no clue as to how to do this so help would again be much appreciated.

            foreach($users as $var => $value)
            {
            $profile = $modx->getObject('modUserProfile', $var);
            $v .= '<option value="'.$var.'">'.$profile->get('fullname').'</option>';
            }