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 email@example.com mfg /FolderToShare account firstname.lastname@example.org rwidx
Then, to create the folder on the other side:
zmmailbox -z -m email@example.com cm /Accounttoshare firstname.lastname@example.org /FolderToShare