This is a really old thread! Robinhood no longer has any incentives to sign others up. Originally, it was invite only, and you needed a referral code to sign up. Now that they're out of beta anyone can sign up.
I really love Robinhood, and I think you should sign up, even though I don't get a bonus. There are no fees of any kind. No commissions, no monthly fees. Here's how they explain how they make money:
https://support.robinhood.com/hc/en-us/articles/202853769-How-Robinhood-makes-moneyThe only thing that perplexes me is that you have to use a smartphone to view your account or conduct transactions. There is no web app, which means I can't do it from my computer. It's worth the hassle for no fees, though.