In my customize_domain.yaml file, i’ve defined all groups to have their control hidden by default. However, I want to have it available for the group.all_lights. What to i put after control: in the customize.yaml file to achieve this?
Try just control:
(With nothing after it)
Hey thanks, that did the trick 
How to use this with a group.set event?
Control (hidden) is the little button in the top right of a group card that controls all the group children, and whether or not you want to include that toggle on the card.
If you’re having problems with visibility of the whole group that’s a completely different thing. Start a new thread with your configuration and what you’re trying to achieve and we can try to help you out 