For what you're asking, without the Roll20 API, the answer is "No" i think. You just gotta go 1-by-1 as GM, open it, hit EDIT, change the permissions, hit SAVE, every time. With Roll20 API (a Pro feature) then maybe/probably yes. Almost Definitely yes, once you do some configuring and pre-work. You'd need to ask in API forum for exact details. As for Grouping generally, which I thought from the subject line, YES you can make sub-folders of Characters just using the folder feature there. But that won't resolve your request to select-and-change the sub-group of Characters.