If we know in advance, we can save a spot for someone missing in a family reunion portrait and add him or her to the group later. Outdoors, we usually photograph in the shade so it's important that the person we're adding later is also photographed in that same quality of light. It will look unrealistic if that person is photographed in harsh, bright sunshine. He or she should also be photographed showing most of the body so the perspective will be similar too.
Here is an example:
