memberOfGroup-List of internal users

==== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.2
- Linux/BSD distribution name and version: Ubuntu 14.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? No
- Related log if you're reporting an issue:
====

Hi,

We are using iRedMail as our school mail server. Because it is a school the users are in multiple mailLists. The membership is now managed in the user entry. Today I've discovered that it is quite easy to fill a mailList with external users using an memberOfGroup-entry filled with the addresses of the user:

http://www.iredmail.org/docs/ldap.add.mail.list.html

Is something similar possible with the internal addresses? Something like this:

dn: memberOfGroup=demolist@example.com,ou=Users,domainName=example.com,o=domains,dc=example,dc=com
accountstatus: active
enabledservice: mail
enabledservice: deliver
mail: internal1@example.com
mail: internal1@example.com
objectclass: mailUser

I've tried it with the objectclass mailUser which didn't work. Do I just have to use another objectclass?