LONDON—Greensill Capital, a SoftBank Group Corp. -backed financing firm, has raised billions of dollars by offering a way for investors to boost yields by helping companies manage cash flow.

Greensill funds long-established companies such as cereal maker Kellogg Co. and Nissan Motor Co., but it also has a roster of lesser-known businesses that inject risk into its portfolio, according to fund documents distributed to investors. These include a private security firm that runs a hotel in Mogadishu, a coal miner that paid Greensill in stock instead of cash and several firms that got more in financing than they generated in revenue.