This is best done by a room configured to be mailable from outside. It will be a folder in Imap.
Using a room as the common e-mail address for multiple users allows all those users to receive mail addressed to that user.
You can then have each user access that room via IMAP or you can set the room as a mailing list and have the room forward messages to each member of the mailing list. To do this you simply add each user to the list of recipients for that room then any message sent to the room will appear in each users mailbox.
If you want mail sent by each user to appear to originate from the common inbox room things are a little more complicated.
If your users are using a client such as Thunder Bird or Outlook you need to alter the settings for the users address in that client. Basically set the email address to be that of the room.
If your users are using Webcit you need to edit their vCard and set their primary email address to that of the common inbox room.