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.

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.