It's not a loosely organized group, it's a label applied (by the right) to many different groups with little to no coordination operating in entirely different places
It may have a different connotation in the US than in Europe. In US media I’ve only heard the term being used recently, in Europe it’s been around for decades.
In Europe, “Antifascist” is a label that people groups use to describe themselves, similar to how other describe themselves as patriot or pacifist. Someone carrying an Antifa flag at a protest is maybe comparable to someone carrying a Gadsden flag at a right wing rally.
Parent comment was flagged but fair point. There’s nobody calling the global shots. I wouldn’t say “little to no coordination” but I think I’m splitting hairs.