If somebody needs the new-posting-notification-function for any user, here is the way to get it.

To allow sending mails to ordinary users
- open the functions.inc.php and delete "user_type > 0 AND" in line 1072

To add the new-posting-notification-function to any new user:
- open the admin.inc.php and add in line 940 "new_posting_notification" under INSERT INTO and the number '1' under VALUE.

The whole line should look like this:
mysql_query("INSERT INTO ".$db_settings['userdata_table']." (user_type,
user_name, user_real_name, user_pw, user_email, user_hp, user_location,
email_contact, last_login, last_logout, user_ip, registered, user_view,
fold_threads, signature, profile, auto_login_code, pwf_code, activate_code,
new_posting_notification) VALUES (0,'".mysql_real_escape_string($ar_username)."', '',
'".mysql_real_escape_string($pw_hash)."' ,'".mysql_real_escape_string($ar_email)."',
'', '', ".$settings['default_email_contact'].", NOW(),
NOW(), '".$_SERVER["REMOTE_ADDR"]."',NOW() ,".intval($settings['default_view']).","
.intval($settings['fold_threads']).",'','','','','','1')", $connid) or
die(mysql_error()); //raise_error('database_error',mysql_error());