SpaceX officially joins the Nasdaq-100 today, and investors holding some funds tied to the index will end up exposed, whether they like it or not. Here's what you need to know:

-- Mutual and exchange-traded funds with $800 billion in assets under management are expected to buy SpaceX shares in order to mirror the index's performance.

-- SpaceX will be a small component in the Nasdaq-100, for now. Even though SpaceX's $2.1 trillion market cap makes it one of the most-valuable U.S. companies, it will have an initial index weight of less than 1%. The Nasdaq adjusts index weights by a company's so-called free-float, or the number of shares available to trade publicly, and SpaceX sold less than 5% of its total shares in last month's public offering.

-- Invesco's QQQ ETF is the biggest fund adding SpaceX, but not the cheapest. A long-running marketing campaign has made QQQ a favorite fund among individual investors, but those seeking the lowest fees now have cheaper options like State Street's newly launched SPDR Portfolio Nasdaq 100.

-- Index inclusion can boost a stock-up to a point. Buying from index funds will be weighed against selling by employees as lockup periods end over the next year-a phenomenon that analysts say weighed on shares of newly public companies like Facebook in the past. SpaceX also won't be joining the most widely tracked index, the S&P 500, for at least a year. Analysts said SpaceX's financial performance and direct demand from investors are likely more important drivers of long-term performance.