Openfire how to lock down in a multi group enviroment

Bit of a strange one but ill try to explain as best I can

My setup is pretty simple I have a single machine running centos and mysql and openfire , The mysql is used as a password database for openfire.I am using pidgin as the xmpp client desktop side

Now my aim is to have multiple user organisations with no contact with each other ( I have packet filtering installed) each of these organisations will have an admin over their rooms who can take care of creating new rooms ect.

Now to explain my main requirement:

assuming I had 2 users and 2 groups

user 1 is within group 1

and user 2 is within group 2

If user 1 attempt to contact user 2 even through buddies -> new im he should be denied and preferably told the user does not exist. ( not sure how to get this on packet filtering as Ive got it down so they wont receive the IMs but can still open the convo if he knows the users username albeit the packets are dropped by the filtering.

As a further point would this be possible if I used subdomains assigned each subdomains a group and set it up so no one knew anyone outside their subdomain exists is this possible ? <-- this way would actually be preferable.

You can’t have subdomains in Openfire.

Not sure if this is possible (for server to say that such user doesn’t exist). What exactly are you filtering with Packet Filter? Try blocking IQs between those groups and see if this changes anything. But i think this could have no result, as search query is operated by a server and Packet Filtering only blocks communications between clients (though these communications still go through the server).