Sketching the face means looking for the structure, drawing accurately enough to capture the individual but without losing the freedom of expression that is inherent in sketch technique. It can be a little bit daunting, especially if you're drawing someone you know, but take your time and observe, the same as you would any other subject. You might find that your family and friends are excited about sitting for a portrait and will patiently sit long enough for a more formal drawing, so have in mind some portrait drawing techniques just in case. And if you're tired of conversation, people will interepret your silence as concentration. Except for the kids, who will probably want to draw too, so take some spare sketch paper. And you can always entertain them with some cartoon faces!