Selection contours

[Ofnuts]

Yes but not in that order...

1. You need to have a clean cutout of the person (in other words, that person on a transparent background). This is typically hard to do if the photo wasn't taken with this in mind (white background or "green screen"). Some tools may help (Foreground selector, for instance), but this is rarely perfect (loose hairs...).
   
2. Open the group's picture
   
3. Duplicate the groups layer
   
4. Add the person's picture as a third layer (File>Open as layers or drag it from the image opened in Gimp)
   
5. Scale it to match. Some color adjustment could be necessary. This is also when you realize that the lighting directions don't match and that you have to find another picture(*)
   
6. Move the person's layer between the two group layers,
   
7. With the eraser (or better, a Layer mask), cut a hole(**) in the first layer through which the person can be seen. This is the other hard part, the people in front of the person have to be cutout rather cleanly and the automated tools cannot be used.
   

(*) The other usual catch is that many of these projects are made starting from a head shot. But in a group picture, you can often see more of the persons in the back rows

(**) Possibly several, around several body parts, see previous note