378. The player who controls the source of the Activated Ability decides when and whether to activate the ability.
378.1. If the source of the Activated Ability is uncontrolled, and it isn't a Battlefield, the owner of the source decides when and whether to activate the ability.
378.2. Some activated abilities specify that one or more players other than the controller or owner of their source can activate them. Those players decide when and whether to activate the ability.