From personal experience, a league is just as likely to drive away new players as it is to attract them. I know when I started I wanted nothing to do with a league – why pay $5 or $10 that I know I have no chance of winning from more experienced players? That’s just throwing away money. And on top of that, it’s “hey, come spend a minimum of $30 to buy a core set, then throw away an extra $5 or $10. Trust me, it’ll be fun.” When I was told that, I felt like I was getting scammed.
For me, what’s worked best is having basic (i.e. Core Set, no influence) decks that expose players to the basic mechanics, because Netrunner has a great set of core gameplay mechanics. Once they try a couple factions and see the obvious holes – “Man, I wish I could bring Mimic or Ninja into Shaper, Pipeline is awful!” – that’s when they tend to be interested in the build-your-own aspect of the game. That’s where they’re likeliest to go for it. The problem is, there’s no fast/easy way to get there besides demoing the game to a lot of potentially-interested people. For every person who joins our local crew, there are several who try the game and go “wow, this is neat,” but never actually jump in.