How do I allow my wife to modify my Google Calendar on her iPhone?

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I have a Google Calendar set up that I use to track appointments for the family, you know things like swimming lessons, doctors visits, parties etc. Works find for me on all my devices.

But I can’t find a way for my wife to edit on her iPhone. She can view ok, but she can’t add or edit, which kind of defeats the purpose of sharing it. She is a Hotmail/Live user and isn’t keen on setting up a new account just for one calendar.

Every guide I can find (that is up to date) talks about sharing to my own iOS device, which I have done, but there doesn’t seem to be anything about fully sharing a calendar to someone else, which in itself might be the answer.

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