The easiest and most fun way I have found to draw people is from life. The reason why is because when I draw people from the vast mix of reality, it makes it 10 times easier to draw people and variations from my imagination. If you look at lots of masters works you will see that they did both. By working from imagination and then refiining the variations from life you allow yourself to build an archetype in your mind. Think of this as a manikin you can pose and arrange in any shape and form you like, once you have collected and downloaded the pieces into your mind and body.
I believe that when you draw something, you not only become that something for a moment, but you actually absorb its energy and personality. When the drawing is complete you have taken in another perspective of that being. Now you can move more freely inside your own mind, with more flexibility and thrust.
If you are looking to learn how to draw people, then the best way is to start in public places like, airports, bus stops, parks, pizza parlors. Setup with your trustee pen and pad. Find a perch that is slightly out of the way of most people and begin sketching the variations of people. If you are feeling rusty or intimidated, start with a blind countour. This technique is when you don't look at your paper when you draw, but you continue moving your pen in the motion that your eye moves across the figures. When you are doing this you have total freedom from all other technical issues that you might use in drawing, like to draw anatomy, or other life drawing techniques.