Right seat flying

This isn't an IFR related question, but I don't see an r.a group devoted
to instruction. I just received the May/June issue of Aviator's Guide
and they make an interesting claim on page 38. In the third paragraph
they say that it is illegal for a non-instructor pilot to fly in the
right seat with a non-pilot in the left seat. I've never thought about
doing this, but never considered that it could be illegal. Can anyone
here confirm this, preferably with a reference to the approriate FAR?
I've never come across an FAR that addressed this before.


Matt