This is a wonderful new feature of iPhoto. I am going through some older photographs in an older iPhoto library, family photos over two or three generations. You pick a photograph of a person and click on the 'Name' icon in the lower left corner of the window of images. You identify that person and then go to the Faces area, select the person, a new screen opens with the photos you have identified as containing that person and below is a group of photos with [perhaps] that person. Click on 'confirm name' and you can quickly click on each image that is, in fact that person and skip others other that are not.
Then, as the program learns the features, you can again hit the 'confirm name' button and new possible matches are suggested, perhaps not as easily matched but some are accurately selected.