How do I add another device to my Apple One Premier plan?

If it is your device, ensure that it is logged in using the same Apple ID as the one that has Apple One.

If you're trying to add someone else's device, add them to your family on iCloud. See this article for more details regarding setting up Family Sharing and adding more members: Set up Family Sharing - Apple Support.

All devices signed in to your Apple ID have access to all the features of the Apple One Premier plan tied to that Apple ID directly.

If you need to add a device belonging to someone else, use Family Sharing: link-> Set up Family Sharing - Apple Support