I checked AgenDAV source code but it looks mysteriously, in the source code I found:
CREATE TABLE shared (
sid int NOT NULL DEFAULT nextval('shared_sid_seq'),
user_from varchar(255) NOT NULL,
calendar varchar(255) NOT NULL,
user_which varchar(255) NOT NULL,
options text NOT NULL DEFAULT '',
write_access boolean NOT NULL DEFAULT '0',
PRIMARY KEY (sid)
);
but AgenDAV is also able to alter server privileges (there are some ACL and PROPPATCH related
functions) - so I am not sure how exactly the sharing works.
What we can do is to add support for ACL manipulation using DAV requests (we will very probably add it in future).