It is very limited to assign a name to multiple images at once.You can try it from the regular browser view instead of "Faces" view:
- Select your project or album in the library inspector and display it in grid view.
- Select an image and press "N" to name the face. Unfortunately, if you select multiple images of the same person, only one will get named.
To speed up naming, it helps to concentrate on one project at a time. Then you can limit the suggested names to that project.
Also, once a few images have been named, Aperture should be able to recognize similar faces and to present them in the "confirm" faces dialog.
If you open the "Faces" Stack of a person, pressing "Confirm Faces" will show a list of similar faces, so you can quickly confirm all other images with the same face.