Basically what the title says: I want to let a guest have control over at least some functions of the lights.
Share with them use MiHome app? Or wait for Yeelight app 3.0 and share it with new Yeelight app.
How do we share with the new app?
Swipe the device card left, there is an choose of sharing. Have a try!
It took me a while to found it. You should add a link in settings menu of each light.