Besides applying transparency effects to single objects, you can apply them to groups.

If you simply select objects and change their individual opacity settings, the selected objects’ opacity will change relative to that of the others. Any overlapping areas will show an accumulated opacity.

In contrast, if you target a group that has been created with the Group command, and then change the opacity, the group is treated as a single object by the Effects panel (the Effects panel shows only one level option—Group), and the opacities within the group don’t change. In other words, objects within the group don’t interact with each other in transparency.

Individual objects selected and set to 50% opacity (left) and group selected and set to 50% opacity (right)