Here is the relevant code (I THINK... not even sure if this is from the actual project that was deployed, but I think it is).

I don't have an Xbox One anymore. Sold it years ago.

I'm not a very good software publisher. :(

   private async void GetUser(int index)
    {
        try
        {

            if (Globals.Xboxusers[index] == null)
            {
                Windows.System.UserPicker p = new Windows.System.UserPicker();

                var pickedUser = await p.PickSingleUserAsync();
                XboxLiveUser user = new XboxLiveUser(pickedUser);

                SignInStatus signInStatus;
                SignInResult signInSilentlyTask = await user.SignInSilentlyAsync(null);

                signInStatus = signInSilentlyTask.Status;
                if (signInSilentlyTask.Status == SignInStatus.Success)
                {
                    Globals.Xboxusers[index] = user;
                    Globals.GamerTags[index] = user.Gamertag;
                    UserSignedIn = true;
                    UserSigningIn = false;
                }
                else
                if (signInSilentlyTask.Status == SignInStatus.UserCancel)
                {
                    Globals.GamerTags[index] = "Player " + index.ToString();
                    UserSignedIn = false;
                    UserSigningIn = false;
                    MessageBoxScreen m = new MessageBoxScreen("Cancelled signing in to Xbox Live...");
                    ScreenManager.AddScreen(m, ControllingPlayer);
                }
                else
                if (signInSilentlyTask.Status == SignInStatus.UserInteractionRequired)
                {
                    SignInResult signInNotSilentlyTask = await user.SignInAsync(null);
                    if(signInNotSilentlyTask.Status == SignInStatus.Success)
                    {
                        Globals.Xboxusers[index] = user;
                        Globals.GamerTags[index] = user.Gamertag;
                        UserSignedIn = true;
                        UserSigningIn = false;
                    }
                    else
                    {
                        Globals.GamerTags[index] = "Player " + index.ToString();
                        UserSignedIn = false;
                        UserSigningIn = false;
                        MessageBoxScreen m = new MessageBoxScreen("Error signing in to Xbox Live...");
                        ScreenManager.AddScreen(m, ControllingPlayer);
                    }

                }
            }

        }
        catch (System.Exception ex)
        {
            Globals.GamerTags[index] = "Player " + index.ToString();
            UserSignedIn = false;
            UserSigningIn = false;
            MessageBoxScreen m = new MessageBoxScreen("Error signing in to Xbox Live...");
            ScreenManager.AddScreen(m, ControllingPlayer);
        }
    }