Putnam Investments employees who sued the company over the allegedly high-fee proprietary mutual funds in their 401(k) plan got a boost of support from the AARP and the National Employment Lawyers Association.

In a brief filed Nov. 8, the groups asked the U.S. Court of Appeals for the First Circuit to overturn a decision absolving Putnam of any liability for including the mutual funds—which earned fees for the company—in its workers’ 401(k) plan. Fiduciaries of retirement plans governed by the Employee Retirement Income Security Act shouldn’t get a “pass” on breaching their duties just because the plan has positive attributes,...