In State of Decay I know that the character I control earns XP. But what about others? Will a character who is with my character earn XP with me or for their actions or do I have to be in control of a character for them to earn XP? Will characters on base earn XP for base activities? Or characters on missions?
To answer your questions about companions and experience, the wiki answers this at the note section which I quote as follows
> AI-controlled survivors, unlike player-controlled ones, do not gain skill experience and thus cannot level up skills. Also, they do not consume ammo or any consumables, neither do they wear out guns or melee weapons that they use. They are also unaffected by encumbrance and never run out of stamina
So while they don't get experience, they do have other advantages such as ammo and weapon conservation