The country being talked about should be mentioned in a post like this. And possibly what you mean by instructing.
With a private licence you can take your friend up and let him fly the plane. You can even teach him to land. That's instruction of a sort.
In Canada in order to give dual instruction that a private or commercial student logs, to supervise a solo student, or to recommend someone for a flight test, you must hold a commercial licence with a flight instructor rating. For instrument, multi-emgine or float ratings, a certain amount of experience is required, but the instructor rating is not.
In the USA the flight instructor qualification is a separate licence, with separate qualifications to teach multi-engine, instrument, ground school, or other instructors.
I would think that in most countries you would need a commercial licence to be paid to do work involving flying an airplane, but I think the UK has some exceptions.