Goldman Sachs Group Inc. earned enormous fees from underwriting SpaceX's blockbuster June IPO, yet the far larger payout tied to that listing was set in motion years earlier.

People familiar with the matter said that more than five years ago, when SpaceX was valued at just tens of billions of dollars, Goldman arranged for high-net-worth clients to invest in the aerospace company. When the company went public at a valuation of $1.77 trillion, Goldman clients who sold shares captured enormous gains while generating hundreds of millions of dollars in incentive fees for Goldman.

Goldman partners were allowed to invest alongside clients, the people said, requesting anonymity to discuss private information, meaning some current and former executives also reaped large payouts. A Goldman spokesperson declined to comment.