Adam d'Angelo[1] who is on the board is also the founder of a commercial AI startup called Poe which lots of people are claiming is a conflict of interest given openAI are moving into a space that seems adjacent/overlapping. Sam Altman said Adam is one of openAI's biggest customers and its helpful to have the customer perspective on the board.

[1] https://en.wikipedia.org/wiki/Adam_D'Angelo

How is Adam one of their biggest customers?

This is not where I originally read it, but it will do[1]:

"A person familiar with the situation said OpenAI's board appreciated having the perspective of a customer like Quora among its directors."

[1] https://tech.hindustantimes.com/tech/news/openai-director-ad...