Can't share my Apple TV+ subscription

On what device is the Family Sharing group member trying to view this?

Being asked to sign up, is a sign that he is either not signed in, or signed in with an account that is not linked to the group that has this shared subscription. Or he’s trying to view content that is not part of this Apple TV+ subscription.

Paid content (not subscription) requires an additional setting of Purchase Sharing, which in turn requires that the organizer is to pay all the billing for the whole group. Purchase Sharing is not needed to share an Apple TV+ subscription. How to share purchases with your family - Apple Support

On what device is the Family Sharing group member trying to view this?

It is your subscription. The Family Sharing group member may enjoy that subscription, but he is not himself subscribed as such. His account won’t show this subscription among his subscriptions.

The Family Sharing group member can just go to the tv+ tab of the Apple TV app to view the Apple TV+ content. All the Apple Originals should be unlocked for viewing. Just be signed in with his own account, as matched for the group setup. No specific settings are needed. Subscriptions aren’t listed in the Library tab, if that is where he was looking.

Family Sharing group members must have an Apple ID account for the same country or region as the organizer. Users with a billing address for other countries can’t be in Family Sharing, and thus can’t access the shared content.