This article isn't a how-to, per se. It's more like a research report written after attempting to build such an app for the first time. There's nothing wrong with that, though, and this write-up is super useful if you're about to tackle a similar problem space.
Open source libraries are tried, facial recognition services are evaluated, and their takeaways are solid, if not a bit disappointing.
As you can see, the really simple idea of using facial recognition functionality was not that simple to implement.
The entire piece is worth a read.