[horde] Hook to show only certain users

[Mark]

Hi,

I have many people logging in via their email address from several domains, 
could someone give me an example hook that is able to restrict users from 
sharing for instance mailfolders and addressbooks cross-domain.

the list of "Individual users" should only contain the users in the same 
domain.

Perhaps the nicest way would be to create an group based on the domain and 
only allow sharing within this group ?
this would allow for an easy loosening of the restriction by adding some users 
to some group if they really want it (although I don't think cyrus allows for 
crossdomain acl's, at least not last time I checked).

Hope someone has a nice solution for this as I could not find an example in 
the hooks.php.dist.

Regards,

Mark.