Ask HN: Founder vs Co-Founder title etiquette

As an example of this, look at BankSimple. Josh and Shamir had been working for several months on the business deals and the beginning of the technology stack. They brought Alex on months later, but all three are now considered co-founders. Obviously, your situation could be different, but if the person's coming on as a partner, and not as an employee, then I'd go with "co-founder" for all involved.

What do you mean by "founds a company"? Have an idea, create a product, incorporate, have paying customers, etc.?

Right - I have trouble with this too. How does person B join an already established, built company as a "co-founder"?

both person A and person B are co-founders.