To create a shared mailbox in Microsoft Exchange 2010 you’ll need to use the Exchange Management shell.
New-mailbox sharedTest –shared –userprincipalname Sharedtest@somedomain.com. Other settings are available for New-Mailbox – http://technet.microsoft.com/en-us/library/aa997663.aspx.
After its creation, notice the different icons for the shared mailbox
Right click the user (shared mailbox) and select “Manage Full Access Permissions
Add the users you wish to allow access.
Note this couls also be scripted with Add-MailboxPermission –Identity sharedtest –user “Lab\tu12” –AccessRights “Full Access”
Then you can log into Outlook Web access (or Outlook) and add them. Here is the process for OWA in Exchange 2010.
You can now see both Mailboxes from OWA.
Hope it Helps