How to indicate that I am an ERC reviewer in CV?
I think it is pretty common to list such reviewing activities in CVs, at least I've seen it often. Usually, people list these activities under "Service", as in "volounteer service to the academic community".

However, I don't think these things are actually very important, especially at your (presumed) career stage: if you are an ERC reviewer, you must be pretty well known, maybe already a Full Prof. (or Associate), thus these details won't make your career any better, in my opinion. They might have more of an impact for junior people, as it means they are recognised as experts by the community, which, with less years under the belt, might be a relatively nice accomplishment.