Ah, so that explains why you enquired about advertising.
Cost sharing is legal, but that doesn't mean that using it systematically to build hours is other than cheesy. If private flying is a hobby, can you think of another hobby in which the participant expects friends who do not share the hobby to subsidise it? If private flying is a route to an ATPL, can you think of another training course in which trainees expect friends to subsidise them? Trainee chefs inviting their friends to dinner and rendering a bill at the end of the evening? Service not included.