If you're an accredited investor, there are actually two YC companies that allow you to do this, FundersClub and Wefunder. I believe both allow you to invest t

If you're an accredited investor, there are actually two YC companies that allow you to do this, FundersClub and Wefunder. I believe both allow you to invest through a fund or individually, though you don't manage the investment personally. Otherwise, you first need a good network to know when these companies are raising money and usually a track record of funding and helping companies succeed or building them yourself. If you're looking for private sale of shares, sometimes they come up on sharespost.