Issue\Request: A customer wants full access to a "multi-user mailbox" that has already been created.
- Log into the Exchange 2010 Server or Exchange 2010 Front End Server and open the Exchange Management Console (EMC).
- Follow the path to Microsoft Exchange > Recipient Configuration > Mailbox and highlight the mailbox you wish to modify.
- Either right click the mailbox or use the side panel to select "Manage Full Access Permission". This opens a wizard (Screenshot 1).
- Click add and find the name of the customer you wish to give full permission to (Screenshot 2).
- Once everyone you wish to give full permission to is added, choose "Manage". This will give full permission to everyone selected.
- Finish and close out of everything.
Technet article with instructions and explanation: http://technet.microsoft.com/en-us/library/bb676551.aspx