At least one of the group members needs to subscribe to Apple TV+, in order to share it.
See your subscriptions overview - Apple
You don’t have to share purchases in order to share an Apple TV+ subscription. Purchase sharing also implies that the organizer pays for the whole group for media purchases. Some groups may not want that.
Apple TV+ gets shared automatically to all members of the Family Sharing group. If they accepted the invitation, then they get access to the Apple TV+ content in the Apple TV app on their devices (e.g. iPhone), when signed in with their own Apple ID account (as used for defining the group). The other group members can just go to the Apple TV app, and start watching content from the Apple TV+ catalog.
The Apple TV+ subscription will be listed for the original subscriber only as an active subscription, but the other group members can access Apple TV+ on their own account. The other group members can see what is shared on iPhone in Settings﹥[Them/Their Apple ID]﹥Family Sharing﹥…
About Family Sharing - Apple
Learn more about what you can do with Family Sharing - Apple Support
Set up Family Sharing - Apple Support
Add a family member to your shared subscriptions - Apple Support
How to share purchases with your family (optional) - Apple Support
About Apple TV+ - Apple
Apple Originals - Apple TV+ Press
List of Apple TV+ original programming (episodic/series) - Wikipedia
List of Apple TV+ original films - Wikipedia
