Let me repeat that again - POSITIVE REINFORCEMENT.
When someone does something, appreciate their contribution. In most startups, the product is a vision of a particular person / group of people and the others are trying hard to materialize that vision. Make sure as the founder, you are grateful that they are sharing your vision and helping you realize it. Anytime you do otherwise, it's a major point against you and will bite you forever in the future.
And while you are at it, the best thing you can invest in before running a company is to understand how to manage work relationships.