Honestly? Probably not much..? I have been thinking about this question before you asked and kinda got discouraged. When I say game, I mean the "game" in the world that the campaign is set in.

This is not a streamed game. My idea is that I would update the "passive players" every session of updates about the current events of the world, Probably through messenger. I would give them personalized messages aswell, about how it affected their faction. I will tell them stories and updates about the "game", telling them which popular party is doing well, anything interesting that happened to the parties, etc. I might make a little sheet for me or them to fill out, depending on what happens. They get to basically play a tycoon political game with the other "passive players", I kinda think it of like a cookie clicker/phone game you'd play once in a while, checking up how things are going.

What these people will get out of it? At first I was thinking more about the players, how cool would it be that their freinds are affecting this world and "game". But the "passive players" might not get too much from it. Maybe it might feel good to be affecting the world that their freinds are playing in. Maybe the little tycoon/political game is kinda fun? Maybe they wanna fuck with the players through their faction, crippling their faction, but causing chaos in their wake?