Ok, these guys with the X's have different contrast and lighting than the guy in the middle and his background.
View attachment 61601
This guy is back lit from a light source coming from behind him, the other guys don't have this.
View attachment 61603
This guy with the X has the opposite light source than the other three.
View attachment 61602
This image would require a ton of work to make it look as if they were all in the image together.
The way to fix this is to find a better background image that contains lighting that they all could share.
Make sure all of your subjects have the same directional light source.
The contrast could be helped with a Curves adjustment layer on the guys that are not part of the original image.