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Near the bottom of the page on the right. Was also on a slide (the one showing new features not demoed or talked about) at the keynote too.
Oh yes now I see "iCloud family storage plans" on the keynote slide!

Finally!
 
If you have a device on iOS 11 you can activate the shared storage plan, all my family’s non iOS 11 devices are now using the shared storage.
 
If you have a device on iOS 11 you can activate the shared storage plan, all my family’s non iOS 11 devices are now using the shared storage.

I assume your other family members paid for iCloud storage - if so how did it go with them cancelling their own plans to just use the family storage or was that all 'taken care of' automagically?
 
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