So stupid, none of Google's docs mention this and support apparently have no clue.
I randomly found it here..
"To set up presence sensing, go to the Google Home app, Favorites tab, then tap the down arrow next to your Home's name in the top left of the screen. Then, press the Set up presence sensing button and follow the in-app instructions."
Submitting this here for others to find as I don't seem to be able to reply to relevant threads.
