Is there a way to change the brightness of a light group without also turning on lights in that group which are switched off?
Groups are intended to work as a group. Which suggests they aren’t a good fit for your use-case. Perhaps a different group with a subset of lights. Without more info 
You can use a template in a script to expand the group of lights and only select the lights that are on. You have to know what you want to change and program it beforehand though. You might as well use a scene.