If you see an accept button (similar to a meeting invite RSVP button), this is how you can accept the shared calendar. Until then, you should see a hyperlink in the sharing email. Click on the accept hyperlink to login to OWA. If you are directed to their inbox (rather than calendar), you need to open the message & click Accept once more.
Note that your calendar will notify you with the same notification settings as the account to which you added the calendar.