Family Sharing requires being invited by a person that sets it up (the organizer). Then you accept the invitation, and you are part of the group from then on, until you decide to leave, or get removed by the organizer. Users can be part of one Family Sharing group at a time, and users can switch groups no more than once a year. Family Sharing group members would need to have their accounts based in the same country.
When you are part of a Family Sharing group AND one of the group members is an Apple TV+ subscriber, then anyone in that group will get access to Apple TV+ through their own account (with the own personal watch history, Up Next queue list, and recommendations based on what else you seem to like). Apple TV+ is not automatically included in Family Sharing. One group member must subscribe to the service.
Family Sharing is an account property of your Apple ID account. It works on devices where you are signed in with that same Apple ID account. It doesn’t matter if you joined the group on Mac, or on iPhone, or on the organizer’s device.
Family Sharing details on Mac can be found in System Preferences. You would find a Family Sharing icon there. This is different from the Sharing icon in System Preferences, which is for sharing the Mac and some of its content – not about subscription services.
On Macs, users need to additionally sign in to the TV app (menu Account) to access shared subscriptions like Apple TV+.
On Roku, users need to sign in the Apple TV app (sprockets wheel icon) to access the shared subscriptions like Apple TV+.
On iPhone, there can be one Apple ID account only, and that is automatically used for the TV app as well.
Family Sharing - Apple
What you can do with Family Sharing - Apple Support
Join a Family group - Set up Family Sharing - Apple Support
Join a Family Sharing group in Family Sharing on Mac - Apple Support
