An investment company formed by two former contestants on “The Apprentice” TV show sold almost all of their 5.5 percent stake in former President Donald J. Trump’s social media company, according to a regulatory filing on Thursday.

United Atlantic Ventures sold roughly 11 million shares of Trump Media & Technology Group within the past few days, after a lockup provision that had barred large investors, including Mr. Trump, from selling any shares ended on Sept. 19.

The two principals of United Atlantic, Andy Litinsky and Wes Moss, were founders of Trump Media and its flagship social media product, Truth Social. They were both contestants on the second season of “The Apprentice,” the reality TV show that helped raised Mr. Trump’s national profile before he ran for president in 2016.

The two men’s entire equity stake consisted of about 7.5 million shares from when Trump Media merged with a public shell company in March and 3.4 million shares awarded weeks later, after the stock hit performance targets in post-merger trading. It would have been worth roughly $150 million at the price the stock was traded on Thursday.

The lockup restricting the sale of shares also applied to Mr. Trump, but the former president said days before the restriction ended that he had no plans to sell any of his 115 million shares. His nearly 60 percent stake in Trump Media has lost billions of dollars in value in the past few months, and is currently worth $1.6 billion.

Since the lockup expired, trading in Trump Media has been heavier than normal, and the stock price has fallen nearly 5 percent over that time. Overall, the stock is down 78 percent since the company’s merger in March with a cash-rich special purpose acquisition company.