Your team, like an orchestra, is comprised of unique, brilliant and talented INDIVIDUALS. It is your job as the conductor and CEO to bring them together and create an environment where they can perform to their optimum. As the orchestra begins practising the conductor will give her musicians sheet music, as the CEO of a startup your sheet music equivalent is a clearly defined culture.
A lot of people in an orchestra probably do have most of the music memorised, but conductors know that they can't rely on an entire orchestra to do that. In the same way, your original team - the first 5 or 10 - will have an innate understanding and gut feel for the culture of the company, but the new members of your team will not.
You need sheet music for that.