Accept a shared calendar invitation on iCloud.com
Invitees to a private calendar must have an Apple ID and use iCloud. Invitees can accept the invitation using Calendar on iCloud.com or the Calendar app on their iPhone, iPad, iPod touch, or Mac.
Do either of the following:
In the invitation email you receive for the event, tap Join Calendar.
In Calendar on iCloud.com, tap the Show Notifications button below the calendar. If you have one or more pending invitations, the button shows a number instead of an arrow.
In the calendar notification, tap Join Calendar.
The shared private calendars you join appear in the Calendar on iCloud.com and in the Calendar app on your iOS and iPadOS devices and your Mac, and in Microsoft Outlook on Windows computers.