With a group of photos selected, choose Find > Faces for Tagging It picks out the faces in all the selected photos, and opens a new Face Tagging window showing a thumbnail for each face. It also opens the tag panel to the right with any "people" categories expanded. When you click on a face thumbnail, you get a larger preview of the whole picture below the tags list. This is a good thing because it will even pick out faces of background people, so there will be some faces you might not recognize. As you tag each face, it is dropped from the face tagging window. You can select multiple thumbnails and tag them all.
I had a few false positives (parts of photos identified as faces that weren't) but not very many. You can select all these and clear them from the window. It also doesn't seem to be able to detect faces that are in profile. Still, this is going to make tagging the people in my photos so much easier! Now if only there was a pet tagging feature!