Digital investing company Betterment LLC is stepping up efforts to attract independent financial advisers to its platform in a bid to expand its focus beyond the mom-and-pop investor.

In the third quarter, Betterment will begin offering a new family of funds to independent advisers, who can buy and sell them for clients on the company’s platform free of commission. The offering, Dimensional Fund Advisors, has $576 billion in assets under management and a strong following among independent financial advisers, said Devin Ryan,...