WeWork Cos. co-founder Adam Neumann has cashed out more than $700 million from the company ahead of its initial public offering through a mix of stock sales and debt, people familiar with the matter said—an unusually large sum given that startup founders typically wait for the IPO to monetize their holdings.

Mr. Neumann, who is chief executive of the shared office-space giant and remains its single largest shareholder, over several years has sold some of his stake in the company and borrowed against some of his holdings, the...