How to share an Outlook 2010 mailbox or folder?
1. Open Outlook.
2. Right click on Inbox/Folder and select Properties.
3. Click on Permission.
4. Click on Add.
5. Type the name on search for the user who will share the Mailbox and click Go
6. Select the user from the list and click on Add.
7. Under permission select appropriate Permission Level
8. Click apply and OK.