First, you will need to move the person into the new group you wish them to be flowed with. To do this, start by going to Images in the top-level toolbar.
Then go to the Portraits tab.
Locate the portrait you wish to move. You can then drag and drop the portrait into the desired group’s folder.
After moving an image from one folder to another, make sure to press Save Changes to save the image to its new location.
Also, if the portraits in that folder have already been flown onto a page, it is imperative that you verify that there were enough portrait spaces to accommodate the new person. For example, if you flowed a group with 25 people in it with 5 rows and 5 columns, everyone would show up but there would be no extra space. However, if you were to then move one more person into that group, the last person alphabetically would be bumped out of the portrait flow. Always be sure to check your portrait flows for completeness and correct as necessary.