Really? I've known dozens of communists, hard socialists and anarchists over the years, and they're perfectly happy to tell you what they believe in. In fact, they are remarkably earnest and passionate about their politics. They start political parties, advertise their activities, groups and societies on posters and flyers around the city. You can pop along and meet them, they're open and happy to talk (they do, after all, want to convince people to their cause). These forums (old and new) have had Communists, who post with Communist avatars and tell you they're Communists.
Maybe it's just that where you live there aren't any, so they may seem quite exotic and secretive.
There are certainly a handful of hard-core activist groups which you'd not be able to easily join, because they do stuff the police would be very keen to squash. There are some leftwing groups that are effectively cults. But even something like Antifa operates pretty much in plain sight, that you could get in with if you were interested with little trouble. In fact, I'll bet Antifa is shot through with state surveillance and undercover cops, because they're so relatively easy to join it barely even would qualify as infiltration.