sideload a modded youtube apk
There is an option to add accounts in the device settings menu. Once added to device, you should be able to switch back and forth between users @ YouTube.
You have to add full Google accounts in System settings to add accounts to the Android TV version of YouTube. Official YouTube apps on other platforms (Roku, Apple TV, Fire TV) let you add accounts thru the YouTube app.
If you want to have YouTube-only accounts within an app on Android TV, without adding them to your system accounts, you have to use an alternate YouTube app like SmartTubeNext.
The weird things is the YouTube app on Amazon Fire TV has an extra screen after you launch the app, where you select a profile. I don't know why that screen is missing from the Android TV version?
Can't people just cast?
Nope, multi user support is non existent. Which is a pain as my account is the main and my YTM history gets riddled with the musical stylings of my 8 years old totally screwing the recommendations