How do you go about receiving representation from one of the Big Four Talent Agencies?

I have a friend who use to always go on those websites which help you find casting calls and whatnot, but they are not having any luck ((most of the stuff online is nonsense)). Anyways, he's a really great actor, so I started looking into some stuff for him ((shhh! he doesn't know yet :p )). It seems that, really, one of the only ways to have any success is to get represented by one of the Big Four Talent Agencies. However, I can't find anything at all on their websites as to how one might go about climbing the ladder to get represented by them. I know that for modeling, even the best agencies have casting calls. So how does this work for acting? Thanks for your help dudes.