[CalendarServer-users] Read-only contacts

Dear all, I would like to provide our staff a shared address book, hosted on a DCS running on an Ubuntu server. Everything works fine so far, except that anyone having access to the shared address book (via a shared CardDAV user account) is able to write changes, even accidentally. I've read that there was some development with respect to shared address books in the past. Can someone please tell me what the current situation is? Is there any path to set up DCS in a way that there is an owner of an address book with write access and other users or groups with read-only access to the same resource? Thanks in advance for any clue Carsten