To access the calendar or tasks of another person you will have to create a new calendar and specify the following field:
Server URL: https://<hostname>/ews/exchange.asmx
Mailboxname: <email address of other person>
Username: <your own windows username>
Domainname: <your domainname>
Select the right folderbase and path for the selected mailbox.
!! Make sure you were assigned the right access rights for the selected folder or else you will receive errors !!