I made hi-res cutouts of each person. The plan is just to shrink everyone slightly and add the new people to the left and right. I'm not sure how it will look, but that's the idea at least.

Also, at first I was worried about matching the lighting in the new photos, but these cutouts are all from different photos and the lighting was quite a bit different between each shot already. I'm no expert, but I just played with the levels in PhotoShop until things looked right. I think it came out looking pretty good.