OpenAI CEO Sam Altman speaks during the Microsoft Build conference at Microsoft headquarters in Redmond, Washington, on May 21, 2024. Jason Redmond | AFP | Getty ImagesOpenAI plans to allow stakeholders to sell a portion of their shares every year, but the company, which has been valued at over $80 billion, is taking a restrictive approach that's raised concerns among current and former employees about the startup's power to determine who participates, CNBC has learned.Due to OpenAI's skyrocketi...