I've never read such a load of bollox as some posts on here. It is perfectly legal for a passenger to operate the flying controls under the supervision of a qualified pilot.
Such an unqualified operator is not in any way acting as flight crew - the qualified pilot remains the legal flight crew at all times and it is up to that qualified pilot to take back control at any time he is not happy with the passenger flying it.
I would proffer a word of caution, though, if you don't know the passenger. many years ago I took 3 girls flying in the Chipmunk (one at a time, of course, in the rear seat). The first two went fine, and each had a go on the stick. The 3rd was noticably more nervous (hence her going last I guess).
As with the first two I let her try the (sensitive, in the Chippy) stick while I followed her through retaining a light hold on the stick in the front cockpit.
She whacked the stick right over before I could stop her, paniced. and screamed and held on with a vice-like grip as we rolled almost inverted. She was easily overpowered and I soon had us staright and level, but if it had been a beefy bloke it might have had a different outcome.