You need to either be or represent the copyright holder. I'm 99% sure most warez groups have no interest in exposing themselves to any legal systems when it comes to what they do.
To whom would I have to prove or disprove my standing to take Rockstar games down, I wonder? Does Steam have any right to refuse such a request just because it's patently absurd?
If you say you represent them, all the scary letter heads will go to _you_.
Of course, you'd probably get counter-sued for a number of civil torts for being a fuckhead.
This is only true of Youtube, where their "DMCA" process is entirely extralegal, which is why there is no penalty for an incorrect takedown request.
If you file a DMCA, and Valve ignores it, you have to take it to court, where a judge will decide whether you prove you are the rightful copyright holder. As in the case with google, a company is free to take down content for any reason, including for bogus DMCA claims.
No requirement such as that is part of the DMCA, and there is no penalty in statute for making illegitimate notices.