Reward Sharing affects users with Basic accounts and means you are splitting the available rewards with users that are close by.
Reward Sharing makes it so Basic users who are in close proximity to each other share the available Geomining rewards. Premium users (Plus, Pro, Master) are not affected by Reward Sharing. If other Premium users are nearby, Team Lift happens which is effectively the opposite of Reward Sharing since it is a multiplier instead of a divider.