by rmsgrey
There are rules explicitly for handling players not being present for all games.The actual campaign is pretty disjointed - rather than a strong running story, each chapter is self-contained, with minimal effect from previous games. On a game-by-game basis, it's a lot more like playing a bunch of games of Betrayal back to back than, say, Pandemic Legacy is like playing a bunch of games of Pandemic back to back.
Unlocks are persistent, but rarely player-controlled, so there isn't really an aspect of planning ahead - yes, if someone dies and leaves a ghost, that changes the tiles for all future games (or at least until the stickers come off), but it's not something you can strategise over beyond the current chapter.
So, at least in my opinion, and bearing in mind we're only about halfway through the campaign, so things could change in the second half, there shouldn't be any problem with having someone only playing sometimes.