OpenAI has completed a deal to help employees sell shares in the company at a $500 billion valuation, propelling the ChatGPT-owner past Elon Musk’s SpaceX to become the world’s largest startup.Current and former OpenAI employees sold about $6.6 billion of stock to investors including Thrive Capital, SoftBank Group Corp., Dragoneer Investment Group, Abu Dhabi’s MGX and T. Rowe Price, a person familiar with the transaction said. That boosted the US company’s price tag well past its previous $300 b...