We can’t find a way for a user to grant access from his own mailbox or mailbox subfolder to other user.
In the meantime, we are giving the necessary permissions through the command line:
doveadm acl add -u prova06@example.com INBOX “user=otheruser@example.com” admin create delete expunge insert lookup post read write write-deleted write-see
Hi Antonio.
You could use an application like EGroupware. This allows you to set the ACLs in your mailboxes:

Stefan
EGroupware Community Manager
Hi Stefan
We are going to test EGroupware in lab environment (maybe we can use some functionalities).
We assume it will work well, but it’s too big a solution for our requirement of simple functionality.
Best regards,
Antonio