I'm quite new to PI3 and I'm sure there is a very easy way of changing a layers opacity but I cannot seem to find it. I just completed the Disintegrate tutorial and I would like to have my foreground fade out as the particles are being moved.
You can't fade out a layer's visibility, if you mean the background footage. The best way to accomplish this would be to create the particle portion of the effect, hide the background, render the particles out and then bring them into your editing program.
In there, you can fade the person out as the particles appear.