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Have you tried your calendar settings to see if you have a default calendar set because you will have Yahoo and the default from Apple.Tried that. No luck.
If you will go to your settings for calendars and under your iCloud calendar you should have events example HOME and if you select that and then go to add an event - Invitees should be there and just did it on my buddy phone at Starbxs where we are and he has your setup.I've tried that as well. Also deleted and added the account back. Other thoughts?