Thank you both for the info. Looks like there's not a way to do this from the web client, but that's okay since it can be done from the CLI with the link that phoenix provided. I will test is out now and see how it works.
One related question, though: When running the command below...
zmmailbox -z -m
share@domain.com mfg / account
user@domain.com rwixd
...can you specify multiple mailboxes, or even all mailboxes for that matter?