As long as they're on the same Google account that should show up in the Google home app automatically once you've signed in. If the device is reset up using different accounts you would have to join the accounts together as a family or reset up one of the devices under the other account.
We had the same problem today. After about 3 hours working on it we determined the following:
I apologize for my late response and thank you for all of your patience while waiting for a reply. I can definitely understand how frustrating your situation must be, but I wanted to make sure that your issue was addressed. Do you still need help with this issue? I know that there's been a delay in response since you posted.
Please let me know if you need further assistance.