You really have to be advertising via a paid advert etc to the general public to get in trouble, since then you are soliciting your services on the basis of a professional status that you do not yet have on a PPL. To take people up in groups as you need the hours, you pay your share (ish) and having put up a note on a company board is a different level entirely, and the CAA probably won't blink an eye - besides would you be happy at that level taking someone up that you don't know?
I remember when I was back at that stage that they give an example such as you go halves with a friend, and as he enjoys it he buys you a coffee in the clubhouse, or maybe a meal later. That will not be a problem, however him walking out and paying your share as well later is.
With regards a family: with respect to most things a family is a single unit - there can be little complaint if your Dad pays the lot every time - not only can you not enforce it as has been said above, but it is hardly for compensation or hire if within a family either.
In short, keep it discreet - you can say an approx cost (not price) that covers their share of the costs, and everyone's happy. Don't flout any more than that an the CAA stays happy.