Unable to invite to HomeKit - Apple ID not registered
I am unable to invite my wife to join the HomeKit network I've setup, says her @mac.com address is not registered with iCloud.
I signed her iPhone and MacBook out of iCloud and then signed them in using her @mac.com address. So she is definitely registered with iCloud. On her iPhone, she uses her Apple ID for iCloud but my Apple ID for iTunes and App Store.
But still when I try to invite her, HomeKit says the Apple ID is not registered with iCloud.
iPhone SE, iOS 11.3