[est31]

Huh, TIL. Investopedia only says "may also include early investors" [0]. I've checked the S-1's of some companies.

Cloudflare's S-1 has a 180 day lockup for "Our executive officers, directors, and the holders of substantially all of our capital stock"

In One Medical's S-1 "We, our directors, executive officers and the holders of substantially all of our equity securities, have agreed" to 180 day lockup.

In the Unity S-1 they have "certain holders of our common stock": "All of our directors and executive officers and certain holders of our common stock and securities exercisable for or convertible into our common stock, are subject to lock-up agreements that restrict their ability to transfer such securities for a period of 180 days after the date of this prospectus", and apparently they allow selling of 30% of the stock within the first two days of trading. Couldn't find any reference on who those certain holders are.

[0]: https://www.investopedia.com/terms/i/ipolockup.asp

[kibibyte]

I suspect it may be the IPO underwriters, the big financial institutions that most companies going public will work with to make their shares available to the public. In short, the company and these institutions negotiate the initial price of the shares beforehand, and on the day of the IPO, those institutions purchase those shares from the company at that price (thus funding the company) and then immediately flip them (hopefully for a profit) to the masses.