To clarify the group members are those who can access the public/shared mailbox. Do not create a distribution list mail alias. Just point to the mailbox!
This is the original feature request I’m talking , which is slightly different from this one:
I think what @pagaille is asking can be done only with a direct connection from postfix to ldap DB, like we did in ns6. But as discussed in other threads cannot be done within the accounts provider of ns7 and upstream postfix version