A family member was seeing only the default 5GB of storage. I prepared to the usual fixes like restart phone, upgrade iOS, leave and restore iCloud and, finally, remove from family and add back in.
Turned out the fix was much simpler as of iOS 16.3.
Settings:Apple ID: iCloud: Manage Account Storage: Use Family Storage.
After I selected this option they got access to our 2TB and the "Use Family Storage" option went away. Instead they had "Change Storage Plan". Tapping that gave the option to leave Family Storage.
Apple's docs mention something about an iMessage going to family members to give them the option to use Family Storage. I suspect that didn't get attention.
Once I'd fixed the problem I used the new keywords to search for Apple documentation. Google found nothing (though it's really bad now) on the Apple site except a post from 2 days ago. So this might be newish.