You will need to create a resource in Active Directory. There is a Resources folder in AD. Right-click it and choose new resource. Ours all start with R-, such as R-departmentfax for instance. After creating it go to Exchange advanced tab in the properties and grant the users rights in the mailbox rights button. Then the users can add the mailbox to their folder list and also create a new profile for the resource in their outlook by going into the mail icon in the control panel, choosing show profiles and then select “prompt for a profile”. Then when they get prompted the next time they open outlook have them choose “new” and create a new profile. They will need to do this if they are going to send mail from that mailbox. If they try to send from their own profile it will go into their own sent items instead of the fax mailbox sent items. Also be aware and notify the users that if they delete mail from the mailbox with their profile open, it will go into their own deleted items, not the deleted items of the mailbox. Either teach them to only delete with the mailbox profile open, or you will have to help each one to make a registry change so the deleted items of the resource stay in the deleted items folder of that resource, not theirs.
You can either use an email enabled public folder or give the users full mailbox permissions to the account then setup their Outlook to open the fax mailbox in open additional mailboxes.