These are Photoshopped. Each actor is photographed separately or in small groups as appropriate. Then each actor is cut out of their original image and placed in an empty background (either photographed or computer generated).
You can see in some cases that the angle at which the subjects' feet meet the ground isn't quite right (take a look at Christina Hendricks' feet, the red-head in the Mad Men shot, and Kiefer Sutherland's feet in the 24 shot), and that there is no interaction between any of the subjects (apart from where they've been photographed together, as in the guy sat down on the right of the Mad Men shot, where you can see the woman's skirt being pushed in by the chair).
Overall it's pretty simple stuff. The trickiest bit is making sure the lighting of the subjects is consistent (both between subjects and with the background: the 24 shot is a bit dodgy in this regard, it looks like the Kiefer Sutherland shot is from a completely different shoot); the Photoshopping is elementary stuff.