Mailbox sharing with symlinks
It's possible to share mailboxes simply by symlinking them among user's private mailboxes. See SharedMailboxes/Permissions for issues related to filesystem permissions.
ln -s /home/user2/Maildir/.Work /home/user1/Maildir/.shared.user2 ln -s /home/user3/Maildir/.Work /home/user1/Maildir/.shared.user3
Now user1 has a "shared" directory containing "user2" and "user3" child mailboxes, which point to those users' "Work" mailbox.
With Maildir++ layout it's not possible to automatically share "mailbox and its children". You'll need to symlink each mailbox separately. With v1.1+ you could do this by using "fs" layout for mailboxes (requires converting existing maildirs from Maildir++).
Doing the same as in the above Maildir example:
mkdir /home/user1/mail/shared ln -s /home/user2/mail/Work /home/user1/mail/shared/user2 ln -s /home/user3/mail/Work /home/user1/mail/shared/user3
One additional problem with mbox format is the creation of dotlock files. The dotlocks would be created under user1's directory, which makes them useless. Make sure the locking works properly with only fcntl or flock locking (See MboxLocking) and just disable dotlocks. Alternatively instead of symlinking an mbox file, put the shared mailboxes inside a directory and symlink the entire directory.