Counterpoint: Hacker News, CDMX EBM scene, goth scene, LA underground art scene, SF renegade rave scene, Latin bars, gay clubs like the EndUp, which have been open and welcoming since 1971, gay black scene, free software movement, gay Asian scene in any city, bear bars, salsa clubs, martial arts clubs, drag bars, anime scene, furry scene, hiking groups, Trekkies, adventurer clubs, punk scene, bicycle riders groups, astronomy groups, ball room dancing groups, central american danza cultural groups, alcoholics anonymous... Even prestigious universities have countless open proseminars where you can just walk in off the street to a graduate symposium. I've gone to many, you really just walk in the door.
I'd argue the majority of cliques and scenes are extremely accepting and people self filter. Having a discriminating door guy is just being an asshole.