Depending on your needs for layer stacking in the timeline there are a couple of solutions.
1) Place an adjustment layer above these layers and add the Transform Effect and animate opacity.
The issue with this method is that the effect will apply to all layers below the adjustment layer.
2) Add an expression control slider to the null object. Add an opacity expression to the first layer by alt/option clicking on the opacity stopwatch and pick whip it to the slider. Select the opacity property and choose Edit > Copy Expression only then select the remaining layers you wish to affect and paste.
Use the slider to control the opacity of all the layers.
CS4 Example: 462_examplesofopacitycontrol.aep.zip
Or you could nest the Comp in another and control the opacity that way….