In the old GIMP I could box select a person then IWarp them without affecting the people on either side.

In GIMP 2.1 how do I use the Warp Transform tool to distort a person and their immediate surrounding background without the warping affecting further parts of the image, like people on either side?

You can use the Cage Transform tool for that. It is in the standard toolbox, thepicto with the multiple selection points connected with lines.

I can't remember if the cage transform pulls in the rest of the image too, but either way, it leaves behind a weird ghost outline of the object you're distorting, unlike the warp tool.

I didn't notice that last time I used it and you can definitely use it only on part of the image.