Final answer:
Athletes and interest groups have taken legal action to improve their rights and change regulations in various cases throughout history.
Step-by-step explanation:
The group of athletes that had to sue to allow agents to represent more than one athlete at a time is not specified in the provided information. Athletes and interest groups have taken legal action to improve their rights and change regulations in various cases throughout history.
However, there have been various cases throughout history where athletes or interest groups representing athletes have taken legal action to change regulations and improve their rights. For example, the passage of Title IX in the United States ensured equal treatment for women's sports. Additionally, athletes' unions and associations have fought for fair contracts and representation in professional sports.