If you have a usergroup permission for that page you can use this:
PHP:
// Check user group permissions
if (!$visitor->hasPermission('Permission_Group', 'Permission_Name'))
{
throw $this->getNoPermissionResponseException();
}
Otherwise you can check his usergroups like this:
Rich (BB code):
if (!$visitor->isMemberOf(4))
{
throw $this->getNoPermissionResponseException();
}
Change number 4 to the allowed usergroup, if you want to add more usergroups replace the number 4 with array(4,3) you can add more groups by adding their group_id separating them by comma.