Arduino in Ground-Chassis (Vehicle): Activate by grounding
You can use transistor to control the current flowing across the LED. For example, by using NPN-transistor, you can do it like this.



The amount of LEDs you can control with this circuit depends on the transistor's ability to withstand current. The amount of current required from Arduino depends on the resistor you are using on the base of the transistor. More about this type of circuit: http://www.sqlskills.com/blogs/paulselec/post/arduino-figuring-out-transistors-and-associated-resistors.aspx