Flagging or Selecting all photos in a Face

How can I flag a group of Faces pictures? How can I Select or Select All of a group of Faces pictures. The choice for any of these is greyed out. I want to create something where I want to include all the pictures with the face of my grandaughter in them. I have named every picture in my photos with her face in it. I choose her face from the corkboard. All her pictures appear. Now what can I do to select them to flag them to work with just all those pictures with her in them? What is the use of going through all this time-consuming process if you can't then work with them? Or am I missing something simple?