So each button is a separate object if you did that. Which is fine. So you just need a way of the game knowing that they are grouped together.
I see you have an 'early adopter' medal so assume you have a full version of C2? If so you can create a family and add all the buttons to it. Then you can just use the action '[family name] -> Set visible -> Invisible'.
If you don't have a full version then you will need to do something like what Telyko said above, but just know when he says variable he means on the object and not a global one.
P.S an instance is when you use the same object by copy one that is already in the layout or by dragging an object into the layout from the project.