Create the public folder if you haven't already and mail enable it, make it visible in GAL for the moment. Set up access rights for your users. Make sure that Anonymous has Contribute rights or it won't be able to receive any mail.
In ADUC go to the Exchange General tab of the users account.
Then select delivery options, do a search for the PF you set up and add it. Select whether or not to keep a copy in the user mailbox.
If you want the other people to be able to reply AS the PF then you'll have to go into Microsoft Exchange System Objects within ADUC and set SendAs rights on the PF for their accounts.
Or why not just get the mail sent directly to the PF and bypass the users mailbox? If original user has left just delete their e-mail address(es) and add to the PF.
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