iCloud: Reply to a shared iCloud calendar invitation
Invitees to a private calendar must have an Apple ID and use iCloud. Invitees can accept the invitation by using Calendar on iCloud.com or by using the Calendar app on their iPhone, iPad, iPod touch, or Mac.
Open the email invitation, then click Join Calendar.
The shared private calendars you join appear in the Calendar sidebar on iCloud.com and in the Calendar app on your iOS devices and your Mac, and in Microsoft Outlook on Windows computers.