Robinhood Markets ($HOOD) is set to join the S&P 500 index effective prior to the open of trading on Monday, September 22. This inclusion is expected to provide a "constant bid" on the stock from passive index funds and institutional investors, potentially leading to long-term price support. While a short-term pop of about 7% occurred after hours, the long-term impact of mandatory buying through 401k allocations and increased institutional legitimacy is highlighted as a significant positive for shareholders.