[Dovecot] Multiple domains with shared Folder - how to?

[Volker Siebelink]

Hello,

I set up Dovecot so that I now have about 20 domains on my mailserver. 
So far everything works fine.
Now I have the need to set up shared maildirs, but they shall only be 
shared for the users of just one domain. What do I have to do?

Thanks in advance for any hint!

Volker