[horde] How to prevent that users see a list of all other users when sharing objects?

[Volker Then]

Hi,

is there a way to prevent that users can see the full list of all  
other users of the system when they set permissions for a share? It  
would be better, if only the list of available groups would be shown.  
Is there a configuration for this? If so, I did not find it.

Thanks,

Volker