Sid Wilson joined Slipknot in 1998 as its turntablist, and would later go on to man keyboard duties for the group as well. Though when he joined in 1998, Shawn "Clown" Crahan really tried to make it abundantly clear that Slipknot was a little less than normal. According to Wilson in an interview with Brass Knuckles, Clown decided to come down off stage during "Tattered And Torn" and tried to fight Wilson. It wasn't just Wilson, as he says Clown did this during pretty much every show, but Wilson does say he got in a few good shots.
Impressed with his willingness to be insane, Wilson says that's why Clown recommended him to the rest of the band.
“Clown was trying to tell me, ‘We’re crazy, we wear masks!’ I’m like, ‘Okay, I got it.’ He kept saying over and over, ‘I don’t think you get it.’ I’m like, ‘Nah, I don’t think you get it. I’m crazy, fool. I got it,’ I go and watch my first Slipknot show … There’s a song called ‘Tattered and Torn’ and Clown comes off the stage on that song. He gets physical with fans — go down to the pit and they’d wrestle with them and he’d wrap a microphone cord around them and drag them across the floor.”
“He decides he’s going to come down off the stage after me. So I just start coming over shoulders of people. He’s right at the front by the monitors and I’m right there and I pull up and I grab his head and just start head-butting him as hard as I can. I head-butt him six times. Oddly enough, that’s his number —six. I was trying to knock him out. [Clown] went back to Joey [Jordison, drums] and he goes, ‘I don’t care what anybody says, that dude is in the band.’”
So there you have it. All you really need to do to join Slipknot is wait for "Tattered And Torn" to come on and then wait for one of the dudes to come out into the crowd to fight you.