Family sharing is through Apple's family sharing program, you can set up your Family through Apple and then use that in Seven for a Family subscription.
You can learn more about how Family Sharing works here: https://www.apple.com/family-sharing/
You can learn how to set up Family Sharing here: https://support.apple.com/en-us/HT201088
To set up Family Sharing in Seven, first ensure you're using the latest version of Seven in the App Store. Then go to Seven > Settings > 7 Club.
If you don't have a subscription yet, tap 'View all plans' at the bottom of the screen. Then, tap 'Family' and choose your monthly or annual subscription.
If you're already subscribed, you can upgrade to Family Sharing by tapping 'Family Upgrade' in your 7 Club settings in Seven. After subscribing, you will then receive a prorated refund from Apple for the remainder of your original subscription.