Nudist Beach is a rebel organization founded by Isshin Matoi. Its goal is to thwart the plans of the Kiryūin conglomerate to subjugate the world to the will of the Life Fibers. As the name implies, its members work almost or completely naked most of the time.
The organization's base is situated in Ōsaka. Along with the other schools of the Kansai region, Kaneo Takarada is a supporter of their cause. They were the ones who gave him his anti-Goku Uniformpowered exoskeleton, the Dōtonbori Robo.
Near the end of the Tri-City Schools Raid Trip, a big part of the organization's facilities were destroyed. However, there still remains a part which was left intact. While it looks ragged from the outside, it has a lot more inside. Fitting to the name, it is a beach.
As the rebels are opposed to Life Fibers, they almost never wear any themselves. Instead, they fight using practical means such as subterfuge, sabotage, traps, and other guerrilla style tactics. They are shown to use technologically advanced bipedal machines dubbed "DTRs", or Dōtonbori Robos, and weapons designed to fight Life Fibers directly; like the special jamming bullet Tsumugu delivered to Mikisugi during the First Naturals Election.
As of episode 19, Nudist Beach has absorbed the Student Council into their ranks.
In episode 21, when Senketsu is torn off of Satsuki, she is seen wearing Nudist Beach's Guerrilla uniform. Though it may just be a spare set of clothing she is wearing, it is assumed she is now a part of Nudist Beach.
In the XP4 fics, the Nudist Beach is allied with the Heroes Coalitions.
In the AWA fics, the Nudist Beach is allies with the All Worlds Alliance.