[horde] Shared categories; howto?

[Volker Then]

Hello,

I want to set up a group that will have an equal set of categories for 
each member of the group. The category colors should be the same for 
each user. As far as I understand this could be realized by setting up 
a "hook".

Is there somebody who did that before? I need some hint to setup this 
specific hook. (I already looked at config/hooks.php, but I can't 
figure out how to start.)

Thanks Volker