WeWork Cos. on Thursday agreed to a $4.4 billion investment from SoftBank Group Corp. and its Vision Fund, part of an aggressive global expansion plan for the coworking startup.

New York-based WeWork, which rents office space to workers, and Japan's SoftBank have been formalizing an alliance this summer that includes a $500 million project in China and a joint venture in Japan. Part of the funds will also go toward letting some shareholders of the privately held startup cash out.

SoftBank is sitting on a massive cash pile thanks to a nearly $100 billion technology fund it started with the Saudi Arabian government. The Japanese investment firm has been ramping up deals with tech companies, rankling venture capitalists in the process. VCs worry SoftBank could drive up valuations and create a frothy market.