Outlook Web App allows you to add other calendars to your calendar view. Depending on the permission levels you have been granted, you will be able to act on these permissions (free/busy, read, write, etc.).
If you've received an invitation to share someone else's calendar, you can select the link in the invitation to add their calendar to your calendar view. You can also go your calendar to add other people's calendars to your view.
1. Click on the App Launcher in the upper left-hand corner and select Calendar.
2. Within the Navigation Pane, right-click Other calendars and select Open calendar.
3. To add a calendar that belongs to someone in your organization (or a resource calendar), enter their name in "From directory:". This box works just like the "To:" box on a new email message. After you've entered the name, select Open.
NOTE: If you receive a URL from an Office 365 user and want to open up the URL via a web-browser, you must change "http" to "https" for the URL to work properly.