Plus can't be shared with Family Sharing. A subscription belongs to the Apple ID that bought it, so each family member who wants Plus needs their own.
You can still use your own Plus on another iPhone of yours:
- Sign in to the App Store on that iPhone with the same Apple ID.
- Log in to the same just streak account.
- Open the Profile tab, tap Settings, then Restore purchases.
Everyone else in your family can use just streak for free with their own account. The free plan keeps one habit active, and includes reminders, badges, dark theme and CSV export. The yearly plan comes with a 3-day free trial for first-time subscribers.
just streak runs on iPhone for now. There's no iPad or Android version yet; Android is coming later.