There's a difference between joining a national socialist group and national socialist joining a group. 

Me and the boys all joined a bunch of established orginizations that are run by church or community groups. 
Far easier to use these as cover and act through them, most wouldn't/ aren't even mad about it. 

Your house is on fire, what the fuck do you care who is in your local fire brigade. 
Arson in my town fell by 60% this year, the fall was so great that we accidentally triggered a federal investigation into why our branch is so effective- we assumed the investigators were glow niggers and it was very difficult to explain why we didn't want our success on the front page. 

Basically I can spot a communist a mile off, half the arsonists were involved with the fire department itself in some tangential way, and we shamelessly off dutied several people we/everyone suspected of arson. 

60% reduction this year in our area,  no loss of life, several fires put out before fire engines even arrived.
The neighboring regions unironically refer to us as storm troopers.