MAD_B

$data = array();
foreach ( ORM::factory('Role')->find_all() as $role) {
    $data[$role->name] = $role->users->count_all();
}
arsort($data);
echo Debug::vars($data);

$data = DB::select(DB::expr('count(user_id) cnt, roles.name'))
    ->distinct(TRUE)
    ->from('roles_users')
    ->join('roles', 'RIGHT')
    ->on('roles_users.role_id', '=', 'roles.id')
    ->group_by('role_id')
    ->order_by('cnt', 'DESC')
    ->execute();
       
echo Debug::vars($data->as_array());