There was an entire website dedicated to militia movements. It was like a marketplace lol. Groups would post their information and recruitment and there would be a grid list, and you would select on to see if you like and click join

https://www.vice.com/en/article/y3gmmk/meet-mymilitia-where-right-wing-extremists-find-friendship-and-fantasize-about-violence

MyMilitia makes its pitch to prospective “militia” leaders and members under a tab named “Why use the militia manager?” It touts an array of features, including an opt-in “member map and locator,” which allows militia leaders to track their members’ movements, voice chat, and photo galleries where users can upload images from meet-ups or training events.

Bet you it’s not even open source lol