By Laura Cooper and Chris Cumming

Blackstone Group Inc. is joining investors in Diligent Corp. in a deal that values the maker of corporate-governance software at north of $4 billion, according to a person familiar with the matter.

The largest portion of the roughly $500 million investment was contributed by Clearlake Capital Group, which increased its previous stake in the company, the person said. The remainder came from Blackstone and some limited partners of Insight Partners, a New York firm that remains Diligent's largest investor.

Diligent makes software that helps boards, committees and leadership teams with secure communication and collaboration, and has more than 19,000 clients. A software-as-a-service provider, Diligent is expected to generate more than $350 million in 2020 revenue, according to the person.

Insight took Diligent private off the New Zealand stock exchange for $624 million in 2016. Two years later, Diligent brought on Santa Monica, Calif.-based Clearlake as a minority investor.

Blackstone's investment comes from the firm's opportunistic investment arm, called Tactical Opportunities, which invests flexibly across asset classes, sectors and regions.

The investment comes at a time of great appetite for software investments by private-equity firms and the institutions that back them. Limited partners have sought new investments in technology-focused private-equity funds in part because of the growth prospects of tech companies and the SaaS recurring-revenue model used by many. Valuations in the sector have held up better than most despite economic disruptions from the coronavirus pandemic.

Private-equity funds focused on technology raised $30.1 billion this year through July 6, the third-highest sum ever through that period, despite a pandemic-related slowdown in private-equity fundraising in general, according to data provider Preqin Ltd.

Insight and Clearlake have taken part in the fundraising boom, with both firms collecting their largest funds yet this year. Insight closed on $9.5 billion for its latest fund, and Clearlake raised more than $7 billion for a vehicle designed to take advantage of market disruptions.

Miriam Gottfried contributed to this article

Write to Laura Cooper at laura.cooper@wsj.com and Chris Cumming at chris.cumming@wsj.com