We've heard of such, and seriously considering joining. We like boondocking, prefer being more secluded rather than campgrounds jam packed in with everyone. Really like the idea of being able to book boondocking sites at so many different places.
I just booked a stay via the other one, hipcamp, for an upcoming trip to Maryland. Like that hipcamp has no annual fees, but of course, you pay for it anyway via booking fees when you do book stays with them.
I did not realize that Harvest Hosts and Boondockers Welcome recently merged. That explains some things, and makes sense. One of these days will probably join.