Hi, all.
I achieved mailbox sharing with Dovecot & Roundcube webmail in iRedMail[1] today, just want to share you my experience, hope it helps. Although it's for iRedMail, but it doesn't depend on it.
http://www.iredmail.org/wiki/index.php?title=IRedMail/FAQ/Share.IMAP.Folder
It describes:
Requirements:
[1] iRedMail - Free, Open Source Email Server Solution: http://www.iredmail.org/
List info: http://lists.roundcube.net/users/ BT/9b404e9e