Assuming everyone is on an Exchange Server, the best way to do that that I'm
aware of is to add the calendar you want to your Folder List. Before that
can happen, you have to make sure this person has given you at least Reviewer
permissions at both the Calendar level (which most people will do) AND the
Mailbox level (which most people don't know to do).
Once that is done, do the following:
1. Go to Tools, Email Accounts.
2. Make sure the radio button is in View or Change Existing E-Mail
Accounts, then click Next.
3. Make sure Microsoft Exchange Server is highlighted, then click Change,
then More Settings.
4. Click to the Advanced tab.
5. In the Mailboxes section click Add, enter in the username of the person
you want to add, then OK (or Next) out.
When you've done this, the person should be in your Folder List and you
should be able to expand their Calendar. I think that *also*adds them to your
My Calendars list by default.