It would be very difficult to give a reward per sign up without the system being gamed and abused. Centralized websites can do it because of KYC.

Then implement a referral system without rewards. See my other comments to this post why I do not believe in indefinate referrals. It'll create pyramids of split earnings and I can tell you, some will be very focused on this and in the end will sit back, do nothing on Steemit and take quite a bit of earnings. I think Steemit should not provide such changes, Steemit will grow even with a rewarded referral feature.