I mean, they sort of didn't.

They were together for a year leading up to the story (mentioned here: "Tattletale ... had one year with the Undersiders [before the story began]", and she was a founding member). During this period it consisted of Tattletale, Regent, Grue, and Bitch, which is by no means a sizable team, and they were especially encouraged to stay together because they were being paid a lot to do not much. They tried to recruit Circus and Spitfire, but failed on both.

Skitter joined on April 12, 2011. She left a little more than a month later, on May 14, and rejoined May 31; by this point Imp had joined the team. By the time Skitter voluntarily surrendered herself to the PRT (on July 14), they were mostly running their own areas, and it seems like they rarely did anything as a group until the Fallen, the Teeth, and the Ambassadors arrived. Up until that point there was quite a lot going on.

During Skitter's incarceration, she learned the Bitch had gone into the other world and was living there full time; so now there were only 4 members of the team again. 14 days after Skitter's incarceration, Regent died during the Behemoth fight, which dropped the membership down to 3. Sometime after this, possibly shortly afterward, Imp killed Heartbreaker and formed the Heartbroken with his kids; it's likely she spent most of her villainous time with them, effectively dropping the membership down to just Tattletale and Grue.

Of course, Tattletale was running things from the background, and Grue was most likely spending the majority of his time doing stuff with the Red Hand (if his dating Cozen indicated anything) or dealing with his PTSD.

The Undersiders were together for the big stuff, because Cauldron called them in together. Other than that, it seems like they rarely interacted in any real capacity, and mostly existed as "the Undersiders" so they could have their claim of Brockton Bay.