It's more like how "Anonymous" was in the 2008 Scientology demonstrations. Just buy a _V-for-Vendetta_ Guy Fawkes mask and you're in. Wanna be in Antifa? Just go to a demonstration and hang-out with other people who simply look like they're in Antifa - though that doesn't give anyone permission to fight or act aggressively: there is a degree of mutual-acknowledgement amongst "members": if someone acts aggressively or fights on the offensive (as opposed to fighting defensively against other people who were aggressors[1]) then you wont' find many future allies to witness in your defence in court.

[1] Yes, I'm aware of how murky that sounds.