When I had the idea I also quickly came up with a game plan to grow a mailing list even before launching.
Advantage was that I already have a mailing list with 1500+ subscribers, oriented towards the fruit juice industry. And wine essentially being grape juice... they were a good target.
On top of that, I immediately mentioned that rootstocks would be a paid-subscription newsletter, BUT the first 250 subscribers would get a free life-time membership.
It worked better than expected and those first 250 subscribers - hopefully - will also be the ones that help spread the word once the first editions become available.