So unfortunately, Global Variables need to be on an Event Sheet outside of Groups. The only way I can think of, to sort of organize global variables. Would be to have one Event Sheet dedicated to only initializing Global Variables. Then, include that Event Sheet on the other Event Sheets.
I usually do an Initialize Event Sheet, which contains some general global variables, along with Groups/Events for layout initialization. If I have a Global Variable for say Player Health, then I will place that on my Player Event Sheet.
You can always access a global variable, even if it is not on any included/referenced event sheet. In fact you don't even need to "initialize" the variables at all, they can be on a unreferenced sheet. You can see them as "hoisted"..
Furthermore global variables can only be on the top level, and not in any other scope, since then they would obviously not be "global" any longer. If you need a variable inside a group (or event) it will be a local variable.