There are two separate concerns to think about here. First, is infiltration where bad actors join the organization. This is a difficult problem to address since it's an issue of trust. You should vet people who join, but there's only so much you can do there. It's valuable to read up on how the Bolsheviks handled this problem incidentally where they created a system forcing these agent provocateurs to provide genuinely useful work to survive.

In terms of communication, I would absolutely do that offline. There is no real benefit to doing any serious organizing online, and people shouldn't even bring their phones or electronic devices when meeting in person. That's by far the safest option. Online places make sense for agitation and recruitment, but not for any serious work.

Might want to pop over to the darknet if you're that worried about anonimity.

You will have the structure to rework your own cybersecurity stack much earlier than you will attract the interest of any authority. It's much harder to build an organization that has even a little crumb of impact than it is to secure your communication channels. Focus on the first.

An organization with secure communication and no impact is impossible to fix. An organization with a lot of impact and insecure communication: it's a few days/months of focused work to fix.

• Use Libreboot or (if you can) Canoeboot.
• Use Linux OS like Debian or (if you can) Trisquel.
• Encrypt your OS.
• Don't use iphone and avoid using smartphone unless it uses Replicant or something like DivestOS.
• Use technology such as sandbox or (better) virtual machine.

I am sorry if my english is bad.