I have recently started managing our high school swim team website as a volunteer.  I published updated their google calendar, which was already embedded in the site and a couple of iCalendar users have reached out with issues 'subscribing' to the team calendar.  One of them wants to know how to subscribe to the calendar and the other wants to know how to unsubscribe. 

I searched online and my understanding is that this is all linked to their own google calendar, but I am not an apple user, so I really have no idea how google and apple play together.  Does anyone have any good resources or links to websites that explains this?