Since version 1.7 SabreDAV comes with experimental support for CalDAV-sharing.
Early warning: Currently SabreDAV provides no implementation for this. This is, because in it's current state there is no elegant way to do this. The problem lies in the fact that a real CalDAV server with sharing support would first need email support (with invite notifications), and really also a browser-frontend that allows people to accept or reject these shares.
In addition, the CalDAV backends are currently kept as independent as possible, and should not be aware of principals, email addresses or accounts.
Adding an implementation for Sharing to standard-sabredav would contradict these goals, so for this reason this is currently not implemented, although it may very well in the future.
The interface works however, so if you implement all this, and do it correctly sharing will work. It's not particularly easy, and I urge you to make yourself acquainted with the following documents first:
Implementing this interface will allow a user to share his or her calendars to other users. Effectively, when a calendar is shared the calendar will show up in both the Sharer's and Sharee's calendar-home root.
Inviting people is not instant. There should be a notification system that clients use to either accept or reject this.
A lot of the documentation can be found in the actual interfaces: