Compartmentalization. Local groups act as cells, and only certain and very limited people have contact with a higher person that “manages” those groups and is in charge of organizing/directing those groups, and those people then report to another tier of people/ so on until you narrow down to a central leadership.

Then you organize cells to work together.

You don’t need everyone to know each other. It’s a liability, is extremely dangerous, and is much much harder to organize. This is how spy agencies, militaries, insurgents, political parties, corporations, and so on have operated for centuries.

You don’t need everyone in the country to join up with you, people will join the bandwagon if it picks up steam. You just need to have a good step nucleus that can quickly incorporate those people. The Bolsheviks for example were not very big at the time of the revolution either for example.

I also never said that you could never use electronic communication. That’s what the compartmentalization is for, it makes it much safer if something leaks or gets compromised.