The FT does a great job in this article of identifying the current market players that help "unicorns" raise funds and provide liquidity to early backers / employees through derivatives and secondary offerings.
Companies such as Airbnb and Pinterest are arranging formal programs that allow employees to sell shares — an important tool for retaining talent in a hyper competitive market. Companies also want to retain control of their shares (and by extension, the composition of their ownership). Meanwhile, investors such as mutual funds and family offices are clamouring to tap into the growing numbers of private tech companies that are waiting longer than ever to go public.