I'm a provisional member of my local SRA chapter. It's not all Marxists, so you'd have to play nice with other ideologies there as well, but I like what I've experienced so far. So far I haven't seen any flame wars between ideologies on there. We all hate capitalism and imperialism. They're friendly people focused mainly on mutual aid while also being pro-gun. I haven't heard about any gun orgs that are strictly for Marxists.
The way it worked for me was I signed up for a paid membership online which got me access to the forums where I then posted an introduction about where I'm from. Someone should reach out to you on the forum to give you more info about the chapter you'd join. In my case they invited me to a discord room where you set up a time for a Jitsi video chat with members, after which they voted in private to approve me and I was then allowed into the rest of the discord channel where everyone communicates. I wish it wasn't on discord and would be on Matrix or something but they said it's the easiest to use and set up at the moment.