When creating an appointment, Zimbra provides a "Schedule" tab that is useful in finding a time slot that is free for all participants. There is a catch however, as far as I can see.
If an invitee is a group (i.e., distribution list), the schedule does not expand out and display the free/busy information for each member of the distribution list. Instead, it assumes the group/d-list has a calendar and displays the group as being free for all slots.
This essentially means that there is no support for groups, in Zimbra Calendar's scheduling mechanism.
Perhaps I am wrong?