The latest version of Google Chromecast uses "ultrasonic pairing"
Read about how convoluted the process is in this article: What You Need To Know About Chromecast’s New Guest Mode
Google has taken device pairing, which is already much harder than it should be, and made it harder.
Google - here's one for free - the right way to do pairing between devices is based on the device location. When my phone is near my Chromecast, it should become the keyboard and touch controller for it. When my friends phone is near my Chromecast, the Chromecast should just know that. Easy, no silly device pairing numbers or audio tones. To make the phone the controller for any device, be it Chromecast, Xbox or Playstation, just use GPS and the Internet. No need for silly audio engineering tricks and convoluted processes.
Devices know when they are near each other - just pair them automatically please. And if a device (like Chromecast) doesn't have a GPS, then just put a phone near it during its setup and make the setup process find its location from that.
"Ulstrasonic device pairing" - some engineering genius at Google is proud of this dumb idea.
Chromecast's ultrasonic device pairing is much simpler than it sounds
Chromecast will use ultrasonic sounds to pair your TV with your friend’s phones