How do I get Mountain Lion Calendar to subscribe to iCloud calendar another user invited me to? I have been sent an invitation to join another user's calendar.  I have previously been sent calendars by this person and have had to view the source code of the e-mail where I found a URL that I pasted into Calendar's File:New Calendar Subscription box.  However, unlike previous times, when I try to subscribe I get an error saying:

There was an error subscribing to the calendar.

The data downloaded from https://www.icloud.com/calendar/share/#... isn't valid.


I have noticed that the previous calendars that I subscribed to had a slightly different e-mail text ("Subscribe to my calendar... You've been invited to an iCloud calendar" vs. "Join XXX calendar? People will see your Apple ID if you join.", as well as a slightly different URL (webcal://p04-calendarws.icloud.com), but they are both iCloud.  I tried replacing https:// with webcal:// to no avail.

I don't understand the part about an Apple ID and iCloud.  I've got a Mac and an Apple ID (why does it want to share that with everyone else anyway?), but I don't have iCloud. I do have Calendar which is where all my other subscribed calendars are, so why can't I just subscribe to this calendar just like all the previous calendars they sent me?  I'm already subscribed to all their other calendars so it sounds like it would be a big pain if I have to go some place completely different just to see this new calendar.

Update:
The person who sent me the invitation said they have no idea what they did differently this time, so if anyone could shed light on that instead so they could simply re-send the invitation in a way that I could subscribe that would be an acceptable alternative.
The issue is that the calendar needs to be made public.  This does not mean that anybody can see your calendar, just that anybody can subscribe to it if they have the URL you send to them inviting them to subscribe.  Calendar does not appear to have the ability to subscribe to a non-public calendar, since you don't have to enter your Apple ID to use it, and iCloud uses this to authenticate that you are authorized to view a private calendar.