Well I have never asked a friend to cost share, simply because I can afford it, and most of them can't.
Were my financial circumstances different I would not have a problem asking anyone who wanted to come flying with me, to cost share. Some have offered, but I have not accepted to date. However I may organise a foreign trip later in the year, and cost sharing will definately be part of that.
If someone wants to come flying (either because you offered or they asked), then you explain to them the risks, the uncertainty of weather etc, and tell them, there and then, about cost sharing. If they are not happy to pay their share of the costs (some will not be able to afford it), then they will probably just say that they will come back to you with a date, and never get back. If they don't get back to you, then just leave it, and don't pressurise them by keep on asking for a date. All this is perfectly acceptable in my opinion, as everybody knows where they stand. This is what I understand happened in Mumraaa's situation.
However, if you offer to fly someone, and only later tell them about the cost sharing, or you put pressure on them to come (never a good idea-cost sharing or not), then it is you who are wrong, not them.
Simple really!!