This is a feature request indeed, because AFAIK that is not possible. Everybody means everybody (for reading anyway) and there isn’t such system where it would mean everybody but excluding group X.
Sure you could move everybody else than group X to group A and then allow only reading for everybody, and posting for group A and again just reading for group X (only to help remember later what the heck there happened). But… that is a bit something, isn`t it?