You can do this from the commandline by going to the account you wish to share, setting the permissions to whom you want to share, and then on the other account create a mount to that folder.
If you want to share the "accounttoshare" with the same permissions to everyone, you can grant permissions to ALL instead of to account.
zmmailbox -z -m firstname.lastname@example.org mfg /FolderToShare account email@example.com rwidx
Then, to create the folder on the other side:
zmmailbox -z -m firstname.lastname@example.org cm /Accounttoshare email@example.com /FolderToShare