Dovecot can support mailbox sharing in several different ways:
Public Mailboxes: Shared mailboxes created by administrators.
Mailbox sharing between users: Users sharing their mailboxes to other users.
Symlinking mailboxes: Quick and dirty way of sharing a few mailboxes.
See Filesystem permissions (in shared mailboxes) for common filesystem related permission problem. Note that these permissions only make sense when using a shared filesystem to enable sharing between users.