You can open a calendar of another user within your organisation by the following process (provided that your account has been granted access to the other user's calendar folder):
1. In Outlook, go to File
2. Go to Open & Export then select "Other User's Folder"
3. From the drop-down menu, select Inbox
4. In the Name field, type the name of the account that you want to find. Depending on the size of your organization, you can simply type the first name, and then OK as Outlook will automatically find the appropriate account.
5. If the account is not found, click on the "Name…" button and search for the account in your Global Address List.