When I ran the most active and popular singles group on the east coast (Meetup verified it at the time while we were having drinks), I took pride in showing that the number of Active members was truly Active. I didn't wait for 6 months of no emails from Meetup to delete a member, I deleted a member after 3 months of not viewing the Group site. If they didn't feel the need to even RSVP for an event after 3 months, then there was no point in counting them as active. If all they were doing was stalking the event emails and not RSVPing, then I didn't need them in the group either, because some events had size limits and we didn't need people showing up who didn't RSVP.
Again, it is a personal choice among organizers. I always felt that it was an honesty thing, to show how well the group was doing.