So I saw this in someone's engagement signing book and I cant find it, but i made a mock up of almost what it is. The only thing is that all of the boxes around the subject are lighter than the ones in the middle. Just a little more transparent. So you can sign around them. Its alot more dynamic than my quick sketch here.
oh I use indesign for this stuff

Create a new layer

Fill with white

Erase, cut out, whatever you want to call it the area you want DARK--you can use the selection tool to do this (or a any other method that works for you. I find the selection tool to be best. The most important thing is elimnate the are you don't want. (You could even make a layer mask)

Reduce the opacity of the white layer to get your desired effect

That's it!

As Will said: add a layer with a lower opacity.
Is this the thing you mean:
Click to view attachment
(My wife and I in Kenya this winter)