From Kolab wiki

Enabling the use flag horde

Please note: This will not automatically install the horde webclient! It will only provide some fixes so that you may install horde.

If you enable the flag, please do so on both net-mail/kolabd and net-mail/kolab-webadmin.

What the use flag will do for net-mail/kolabd:

* The horde.schema will be installed into the LDAP schema directory
* The slapd.conf.template will be fixed to include this horde.schema
* The httpd.conf.template will be modified to server content from /var/www/kolab/horde

What the use flag will do for net-mail/kolab-webadmin:

* The webadmin frontend will be fixed so that all accounts that are being created will include the hordePerson object class. Without this fix you would not be able to administrate your users with the webadmin tool after they logged into horde the first time.

Now you can manually install horde into /var/www/kolab/horde and configure it for your system. I would say that this is not trivial but there are some hints here.

Installing Horde as webclient

The overlay provides modified ebuilds to install horde as webclient for your kolab server.

{{{ emerge horde-kolab-all kolabconf }}}

This should install and configure all necessary modules. You only need to run {{{kolabconf}}} to finish the installation.

The horde installation is still completely experimental, contains a lot of very, very bad hacks (and I mean that) but on the other hand it gives you a wonderful groupware server system. So it's probably worth trying it. People that are fluent in PHP: Fixes are very, very welcome.