The Undergrounders are a group of railway track workers who, after they got lost, formed a new society known as Subterranis. The named members of the group include a pipe-fitter named Scott, a pipe-maker, and Dave, the leader. The primary member is just credited as "Foreman".
Mildred the milk carton (post-"The Undergrounders")
Diogee helped them find a way out of the tunnels after a Murphy's Law related incident causes Milo, Zack, and Melissa to find them living below the city. While the majority of the members chose to go back to their old lives, Scott stayed underground. ("The Undergrounders")