It is not by invite only, you can win your way to a ticket by winning the trials. How good do you have to be? Pretty good, not world class to win a trial (some of the guys I know have won regional trials aren't what you would call world calibur guys), but you still need to be excellent to even get there. To actually win- well you have to be the best no-go grappler in the world at your weight that day.
Pretty much all of the guys that win ADCC are BJJ based grapplers. It hasn't been won in any weight class by a non-BJJ blackbelt in over a decade. A couple wrestlers have won, Mark Kerr being the most famous example, and I think that is pretty much it in terms of styles. Basically- 99% of winners are BJJ stylists. A couple guys like Lister and Monson are BJJ-Wrestler hybrids.