ICS invites are added to calendar, but no acceptance sent I am working with iCloud, iPhone and Macbook Pro on Yosemite. 

I receive ICS calendar invites via mail from a user using Outlook. When I click on the ICS file in the mail, it opens and I can add it to my calendar without a problem. It also syncs nicely on all my devices.

However, the sender of the invite does not get a confirmation, in the meeting properties I'm shown as "did not reply". When I open the invite in my calendar, I have no option to accept or send a notification. 

In short: ICS invites are added to my calendar but the organizer does not receive a notification. 

How can I send an accept notification?
Open the Contacts app, select your own email address in the list of contacts. In the menu on top of the screen, go to "Card" -> "Make this my card". Ensure your email address is entered as part of your card. Reload calendar. Then it should work, you will get a notification popup with accept/decline buttons. (I just had the same problem, on a Mac running Yosemite)
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