Depends on where you live, but try to find an organization active in your town whose ideas are somewhat in line with yours. If they’re on a social media you use or have a website with an RSS feed you can subscribe to, use that. Otherwise, try and get in their contacts (many organizations have a contact list broader than their actual member list, so you don’t have to adhere if you’re not sure to commit). Then you get notified for every action they take part in.

To find them in the first place… Keep your eyes peeled for stickers or posters in the streets; and try to find the people at a protest or event said posters advertised.