Thanks for the reply. It seems they’ve changed the way setting up Family Sharing works - there are now four options presented:
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When I select iCloud Storage sharing, and try to send a message to the person, it gives an error message:
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Unfortunately, my iTunes Match (App Store and iTunes account) are different than my main iCloud (which the storage is connected to) - this was of course a clumsy workaround to the sharing problem back before Apple came up with Family Sharing. But it means that I can’t try setting up “legacy” sharing and then adding the storage afterwards, as The accounts are different.