They are very much organized. They have an "officer corps" and "enlisted troops." The "officers" have knowledge of the larger organization, but the average street-level brick-throwers do not. The enlisted are drawn from the ranks of bored college kids, anarchist squatters, drug addicts and others. When someone joins the group and demonstrates talent, they may be tapped to join the officer corps, but there's lots of vetting even at the lowest levels to stop infiltrators.
Naturally, being a wealthy college student makes it much more likely that you'll be considered for promotion to "officer" than if you're a methed-out crustpunk; the latter type of people are the cannon fodder. There was a tweet by someone during the Summer of Love that said something like "I just got arrested with my friends, I got out but they're still in there because they don't have backing. Sucks." The upper tier of Antifa members are the ones with organizational backing who have a better idea of the extent of the organization.
At the national level, they're joined at the hip with the National Lawyers' Guild. They have a lot of academic and media support as well thanks to their elite ties. And I heard a rumor - no confirmation on this - that they actually have a training facility near Austin that nobody unaffiliated with them can get near.