When you add another light bulb to a circuit does it effect the voltage?

When a bulb is added in parallel to a circuit with a single bulb, the resistance of the circuit decreases.

This is because the existing bulb's operating point remains the same, due to the fact that Kirchoff's voltage law states that the signed voltages around a series circuit must add up to zero, with the two bulbs being their own series circuit. A consequence of this is that the voltage across both bulbs must be the same, i.e. that the voltage across the first bulb does not change.

The second result of adding the bulb is that the current in the overall circuit increases.

This is because the second bulb must pull some current in order to operate, yet we know that the voltage across the first bulb did not change. As a result, due to Ohm's law, the current through the first bulb did not change. And, finally, since Kirchoff's current law states that the signed sum of the currents entering a node is zero, the addition of a second load in parallel with the first load must, therefore, represent additional current.