In a previous post Produce 101 Season 2 contestant Lee Seo Kyu (born in 1998) revealed that he was physically attacked by three of his former group mates while promoting in Pure Boy. The article and sources stated the surnames of the members to be Yang, Jeon, and Hwang.
According to Soompi
"Previously, in September 2016, three members of the group Naughty Boys (formerly known as Pure Boys), Hwang, Jeon, and Yang (only referred to by their surnames), were charged with assaulting their team member Lee Seo Kyu during a time when they were still promoting as Pure Boys, kicking and punching Lee Seo Kyu in the chest and stomach.
It was later revealed that the three members were fined for their actions.
Member Hwang was also investigated by prosecutors for repeated assault of another group member, while member Yang was investigated for threats and extortion of a group member.
Naughty Boys came to be when the group, which had been promoting as Pure Boys, left the agency, dropped the name, and began to work independently as Naughty Boys, which resulted in the group being slapped with civil and criminal lawsuits"
Well thanks to internet sleuthing the names of the members of the obscure group were found.
The men are:
Jeon Sangmin, aka "L. Min" who is the leader of Naughty Boys. Born in 1990.
Hwang Taegeun, aka G.O. Born in 1991.
His pictures are tagged #황태근 by his fanbase on twitter. 황= Hwang. 태근 = roughly Taegeun.
Yang Jaewon aka "J-One". Born in 1996. He ended up leaving the group
To ensure I didn't falsely accuse guys, I found the names of the other members and they do not have the same surnames.
The others are:
Park Jinseong aka Jaeha
Choe Issac (left the group)
Shim Youngbo aka Taehan
Moon Sangyun (left the group)
There were other members of Pure Boy, but they never were in the line up for Naughty Boys and the article states the men were in the group. You can see the original nine member line up here.
Just to make sure I checked names that could be traced and they checked out.
Edit: Looks like someone in the comment section was able to find the profile with all the official information here.
kpop info 1
kpop info 2
aurora Puffin Naughty Boys Profile
instagram naughty boys profile
So they were all significantly older than him. Welp, they live up to their name.