My understanding is that sharing of costs is perfectly legal, providing the P1's share is not less than any other person on board. The debate then is "what are the costs?". If hiring, its simple - but when a/c is owned, how do you factor in fixed costs such as hangarage, insurance etc? Or should they be ignored, and only the direct operating costs factored in? At least that would clearly demonstrate no profit motive, and its what I do in practice.
SteveR's point was (I think) about advertising, not sharing. As I recall some time ago when the rules were relaxed there were published guidelines that said it was OK to "advertise" within the aviation comunity, such as on the flying club notice-board (and I guess PPRune is an internet equivalent). Posting an ad in the local paper would clearly not be acceptable!
Slip