Damage from a given individual or group only counts towards a kill for 10 minutes since that individual or group's last damage to the entity - after this period their damage contribution is removed. This avoids the list of damage contributors growing excessively large for an entity that does a lot of combat but never dies.
EXP sharing within groups is unchanged - the difference is simply that the input to this calculation may be less than 100% of the base EXP reward for the kill if other individuals or groups contributed damage.