To view a calendar that belongs to someone in your company, you do not actually need an invitation as the view level of access is given to all internal users by default (though, it can be changed by your administrator or IT persons).

Here are the steps to add a shared calendar to Outlook:

  1. From your Calendar folder, go to the Home tab > Manage Calendars group, and click Add Calendar > Open Shared Calendar.
    Opening a shared calendar in Outlook
  2. In the small dialog window that opens, click Name…
    Click Name.
  3. In the displayed list, find the user whose calendar you wish to add, select their name, and click OK.
    Select the user whose calendar you wish to add.
  4. If you've selected a valid person, his or her name will appear in the Name box, and you click OK.
    The calendar owner name appears in the Name box.

That's it! The calendar of your colleague is added to your Outlook under Shared Calendars:
The calendar of your colleague is added to your Outlook.

Notes:
  1. If an internal user has shared their calendar directly with you, the calendar will open with the permissions they provided; otherwise – with the permissions set for your organization.
  2. To open a calendar that belongs to an external user, you will need ether an invitation or .ics link.