There is no way to sign up for Venetian Society - you get enrolled after your first cruise. However, if you find a current member, they do have a referral program where you both get a cruise discount (I can't remember how much it is - maybe $250 each).
Silversea is definitely worth it if you value the excursions. If you don't care about them, the value proposition is lower.
We only have one SS cruise under our belts, but will be back.