You can practice sketching people “in the wild”, working quickly with just one color of pencil. No way to be sure that your subject will sit still, so you need to work fast! Get the biggest, darkest shapes and shadows in first. Then add smaller bits or lines as needed.
Decide if you want your people to be the only item, or the most important elements. You might want to sketch a scene wheree the people are minor characters. Even small figures can help convey the feeling or atmosphere of a sketch.