To tag a person, select a region of the image by pressing the left mouse button in one of its corners and dragging the pointer across the desired area. A rectangular frame will mark the selected region. Use the toolbar to select an existing person or add a new person to be associated with the region.
To modify a region, first make it active by clicking inside the region. Move the mouse pointer to one of the borders or corners of the selection box. A grabbing handle will appear that you can drag to modify the shape of the box.