I have an app where the user undergoes a sign-up flow. During that flow, the user needs to enable "family sharing" on iOS which involves going into the iOS device settings and completing a series of steps. After they complete this step, they need to continue the sign up flow on the original app. How can I make sure users know to return to the original app after they enabled "family sharing"?
Invite the user to turn on Family Sign-Up immediately after the main sign-up flow on Device A is complete. Because it's not required to finish registration on Device A, you'll see more successful sign-ups (for the reason you're getting at in your question, which is that when users leave an app, they might not come back, for a variety of reasons that you can't control.)
As soon as the main sign-up flow is done: "Congratulations! Welcome to [app]. Let's add your family. Next, you'll be taken to your Family Sharing settings. Turn that on."
If the user turns on family sharing and doesn't come back to the app - they're still done with the tasks they needed to complete.
If they don't successfully turn on family sharing, the app can detect that and invite them to try again the next time they visit the app.
Either way, you don't risk losing the main account sign-up when the user leaves.
What about Device B? Well, they've downloaded the app, and they'll either want to create a new account, or sign on to a family plan. That's a pretty easy thing to offer on the first screen.
- If Device A already added them as a family member, all they'd need to do is connect to that login.
- If Device A didn't successfully add them, they could see a friendly message like, "Are you waiting on a family member invitation? Enter their mobile number, and we'll send them a link to connect."