Experience is granted for defeating opponents, overcoming challenges, completing quests, and as a reward for other appropriate encounters. Earned experience is split evenly among PCs and participating, allied NPCs.
PCs receive a full share of the experience, even if the player is absent that session.
Replacement PCs (whether through retirement, character death, etc.) join play with the same experience as the other PCs.
Effectively, every character should have the same amount of experience at any given time.