Use the marquee or the lasso tool to select something that can be deleted in the group shot, then hit delete on the keyboard. You should see a checkerboard pattern (transparency) in this locus.
If this is not practical, then extend the canvas via Image>resize>canvas size to get more "real estate."
Now place the layer with the image that you have isolated below the layer with the group, position and resize as necessary
Depending on the lighting of the two images, you may have to make adjustments with brightness/contrast so that it does not look fake.
Feel free to post the two pictures here for additional targeted guidance.