If he's really a cofounder, meaning you haven't even incorporated yet, and he has the same qualifications and is going to work as hard as the other founders, you should give him pretty close to the same amount of stock.

So that's actually an easy question. The harder question is how much stock to give to your early hires.

But what if you have already done the site, got 1000+ users, got some press etc.

Yeah. Ok. you haven't got incorporated yet, but based on the man hours spent, isn't it unfair to give the late-coming guy 50%?

Well, in that case you might want to give the existing work a certain value (say 30%), and split the rest evenly.

Weird question. How much value is he going to bring in? How critical this guy is for your organization?

Do you expect us to decide for you? :-) Okey. Give him 43.75% and a written guarantee of a personal parking spot (a-la Mr. Lumberg in "Office Space") when you become next google :)