Granting Access to User Mailbox

Use the EMC to grant Full Access permission for a mailbox
You need to be assigned permissions before you can perform this procedure. To see what permissions you need, see the “Permissions and delegation” entry in the Mailbox Permissions topic.

In the console tree,
– navigate to Recipient Configuration > Mailbox.
In the result pane, select the mailbox for which you want to grant Full Access permission.

In the action pane, under the mailbox name,
– click Manage Full Access Permission. The Manage Full Access Permission wizard opens.
On the Manage Full Access Permission page, click Add.
In Select User or Group, select the user to which you want to grant Full Access permission,
and then click OK.

Click Manage, on the completion page, the summary states whether Full Access permission was successfully granted. The summary also displays the Shell command used to grant Full Access permission.
Click Finish.

http://support.sherweb.com/Faqs/Show/how-to-add-another-persons-mailbox-to-your-outlook-2010-profile-exchange-2007

https://technet.microsoft.com/en-us/library/aa996343(v=exchg.141).aspx