To share a mailbox in the Office Mail (Exchange 2007) environment, with permissions such as Read & Write, you can use the following guide. In the guide, the mailbox to be shared will be referred to as Mailbox A. The user who is given rights will be referred to as Mailbox B.
1) Log in to the Outlook profile of the mailbox which you wish to share (Mailbox A)
2) On the left hand side, select the mailbox (top-level) and right-click => “Change Sharing Permissions”
3) On the following screen, select “Add...”
4) From the following screen, all users (mailboxes) for this domain will be displayed. Select the user(s) who should be given access to this mailbox (Mailbox B). After adding them – by double-clicking or selecting the user and choosing “Add” at the bottom – choose "OK"
5) Log out from this Outlook profile and log in to Mailbox B (the user you have given rights to access Mailbox A)
6) Right-click the mailbox on the left hand side again (as in Step 1), select “Properties”. Choose “Advanced”
7) On the following screen, choose the “Advanced” tab. Choose “Add” in the top half of the screen. Once you have clicked “Add”, you will be prompted for the Mailbox Name. Enter the Display Name of Mailbox A (the mailbox you wish to share)
8) Close all Dialogue Windows with "OK". Restart Outlook. The mailbox will be visible on the left hand side, towards the bottom.