He didn't just convince people. For example, in one of the murders he drove with his accomplices to the crime scene.
Prosecutors can use any concrete action, no matter how minor, to tie him to the murder. Manson's gun was used in the Tate murders, which is more than enough. But even giving the others a place to stay can be enough.
The elements of a conspiracy are (a) planning a crime with someone, and (b) taking any step, no matter how minor, to advance the plan.
Both are necessary. Planning a violent revolution without taking any concrete action is just talking shit, which is generally not illegal. Good thing for Lemmy users.
Likewise, inadvertently helping a murderer without having a criminal plan, like Ma and Pa, is also not illegal.
Put the two together, and you've got an illegal conspiracy.
Yeah I get what you're saying now. The gun and the place to stay aren't enough on their own. That's not what you said earlier. Likewise trying to conduct diplomacy as a private citizen is both A and B. The conspiracy to interfere with the elected will of the people and actually doing so.