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.
READ THE FUCKING MANIFESTO!