ANTIFA is not an organization. It is a tactic, like the Black Bloc, and a very loose alliance.
It's also been around for many decades in many countries. All the propaganda about it right now – from left and right sources – is baffling because of how ill informed it is.
One group of people saying they are ANTIFA can behave one way, and another group of people saying they are ANTIFA can behave another way. In short, anyone who is anti-fascist can label themselves ANTIFA, and people who just want to be thugs can do horrible things and say they are ANTIFA. Basically, anyone wearing all black and doing whatever they want can say they are ANTIFA. You could say you're ANTIFA and go out and smash people.
Edit: I'm not ANTIFA. Anti-fascist, but not ANTIFA.