If you're an "accredited investor" (basically == a million dollars to invest, or a high enough income), you go to someplace like SharesPost or ForgeGlobal, or find someone on this forum who is already connected to the private equity raises, and try to get in on the next funding round. SpaceX will warn you that they will never pay a dividend, that they are quite possibly never going public, and will explain that you can sell part of your stock back TO THEM every six months at prices of THEIR choosing, though they can discontinue this at any time they like; and you can make a private sale but they have right of first refusal to buy your stock at the price of their choice. So, it's super-illiquid. Deliberately. Tesla could delist and still be a lot more liquid than that!