I have a number of user that are missing the "Anonymous" user from the
permission tab of the properties of their default Outlook calendars.

Consequently, their free/busy schedules are no longer visible when they are
invited to meetings.

How can I re-enter this "Anonymous" user? I don't want to go through
re-creating all new mailboxes just because of this.

Microsoft provides a "Remove" button but does not allow re-adding this user.

Your help is appreciated.