If you're thinking about signing up today though, the Signature membership is the one you want. That gives you more serious discounts, as well as free (nearly free) empty legs.
Honestly I wish they'd just focus on the empty leg deals and the folks already doing shared charter operations; I'd easily pay $5K a year for 3-4 empty leg operators and a discount on ad-hoc purchases of JetSuiteX, AdvancedAir, and maybe SurfAir. They seem pretty focused on this 'crowdfunding' idea though -- and from what I can tell, with three decimal points of precision: 0.000% of the crowdfunded flights actually fly.