foreach($oldRoleUsers as $oUid) {
    $this->_deleteUserFromRole($oUid, $role->getId());
}

foreach ($roleUsers as $nRuid) {
    $this->_addUserToRole($nRuid, $role->getId());
}

protected function _addUserToRole(){
    $user = Mage::getModel('admin/user')->load($userId);
    $user->setRoleId($roleId)->setUserId($userId);

    if( $user->roleUserExists() === true ) {
        return false;
    } else {
        $user->add();
        return true;
    }
}

protected function _addUserToRole(){
    $user = Mage::getModel('admin/user')->load($userId);
    $user->setRoleId($roleId)->setUserId($userId);

    $user->add();
    return true;
}