The person who noticed that the Proud Boys were coming to town and rallied people to a counter-protest? Definitely a leader

Nahh you got that wrong. What usually happens is that a lot of people who are into politics (which left-extreme people often are) hear about this at the same time (through some press release, some proud boys twitter account who's rallyin their followers, etc.).

From that point the information spreads over friendsgroups, small discords, tweets, whatsapps, in person, slowly but steadily.

Any left-extreme person who hears this immediately thinks "I'm mad, I wanna show those guys that they're not welcome". Granted, some of us think about much more extreme things, but back to the point. The first reaction from that thought is often "is there a counter protest?". People are then doing the same thing but the other way around, as now everyone is trying to find some tweet, event, whatsapp message screenshot, whatever, of someone saying where the meeting point for an event would be. If none are found, someones gonna create something, which is usuqlly someone who's got a lot of connections with other left-extremists. Often there's multiple people creating the same counterprotest, which gets super messy at times, but somehow everyone manages to meet up in some general spot.

Worst case you just have a bunch of friends groups going to the meeting spot of wherever the initial event is happening.

That's "the antifa". A massive network of friends and friends of friends of friends who are all pretty aligned in their political views (which is "fuck Nazis") but who often don't know more then 5 other antifacists.