Itazura 4 ~Ore-tachi no Sentou Sharyou~

Main character is a molester with multiple arrests. His jail time really decreased his enthusiasm. One day he witnesses a group of molesters on the train who coordinate their actions on an internet bulletin board. With a group of four people it's much easier to conduct mischief and hide the traces of it, so protagonist joins them.

[From vndbreview]