digiKam allows for either manual or automatic face tagging.

For more information about face tagging using digiKam's face recognition engine see The Face Management Tool and People View.

For tagging people manually, you can preview an image, click on Add a Face Tag icon highlighted on the screenshot below.

And then draw a rectangle around the face while holding left mouse button followed by entering the person's name and pressing Enter.

Instead of clicking on Add a Face Tag icon you can draw a face region while holding Ctrl key.