In our road association, each of the members' deeds include an easement over the road. Since it's an easement, not property ownership, it doesn't add to the property tax assessment. Since this is not yet on your members' deeds, it could be granted to each of them separately as an easement "in common with others and with those who may in the future be granted a like right."
There is also a small subdivision off the side of our road. Their road is not included in the responsibilities of our association, although the owners there do have to be members of our association because they use our road to access theirs. A few years ago, the owner of the subdivision sold his last lot, but oddly enough, he then deeded the road itself to his wife. When I wrote and asked them if that meant his wife remained a landowner who owed dues to our road association, they wrote back and sent us a copy of the town tax records showing that they no longer own any land. I'm not sure exactly how that works - that might be a question for your town tax assessor or an attorney.