I think if it's a question of taking mates up, cost sharing isn't gonna be a problem no matter . .who pays or how you go about it even if they fork out for all of it coz they want to do it desperately or something (example given by someone earlier I think.)
The rules are there surely to stop people advertising in the paper and ripping people off and abusing their PPL.
That's not to say the rules are not to be adhered to at all times of course...
Phew.