[Dovecot] Shared mailboxes

Hi..

can anyone give me a hind how to setup shared mailboxes. I've already created the shared and private namespace.

But ive no idea how to share a mailbox. Thunderbird and most other Clients seem not to support the setacl command.

so do i have to create a plaintext file with "user=theldapuseriwanttosharemybox rw" ?

thx Hans