The "All Drivers" section shows the searchable list of all the fleet drivers, who were invited by the fleet manager (through the team management section). – The ones who have accepted the invites and registered with CarbookPlus, can be added to a vehicle or removed from the fleet.
Under "Actions", when you click on "Add to vehicle" option corresponding to a particular driver, the "Assign Driver to Vehicle" option is seen. It displays the list of fleet vehicles. Click on the "+" icon assigns the driver to the vehicle.