where to find hard mode challenges

Hard Mode means going for some amount of days without watching porn, masturbating or intentional (cos wet dreams don't count) orgasm. Also, you can still have sex as far as you do not finish it with orgasm. So that's Hard Mode. And to participate in it you simply need to make a commitment of abstinence for your set amount of days (most popular between people are 90 days, or sometimes 30 days), then you just do it. No need to sign up for anything. You can if you want to tho. There are occasionally group commitment threads, like this one, posted in forum's Rebooting section.