I think it is open to everyone, not just people on their (open) mailing list. The letter says:
Barrel tasting, BBQ, music, bocce ball, horse-drawn carriage rides and more. $15/person.
Pretty cheap for what it is, I think they are just trying to partially cover their costs. Landmark is right next to St. Jean in Sonoma and if you are interested you should e-mail
ryan@landmarkwine.coma