It’s allowed pretty easily.

They had had tender events (where you can sell your private stock super easily)

They've changed the laws recently which makes it far easier - I believe you'd still need to be accredited but for most of HN, that's a low bar. For OpenAI specifically, they've allowed employees to participate in the funding rounds and they did a separate tender offer with Softbank to provide liquidity to early employees as well;

https://fortune.com/2024/12/17/hundreds-openai-employees-10-...