In WebMail enter the account/email address of the User whose calendar you wish to view, in this example "user@domain.com"

and click the View button

Provided the other User has shared his Calendar and you have the appropriate privileges the User's calendar will be displayed.

To add a shared Calendar to the Shared Calendars list (below the View button), go to Settings -> Calendar Settings and add the Users to the Shared Calendars box, like this -