hamlinje, this is very much like calling a bunch of friends/family and inviting them to lunch. It can be touchy, if you are doing the inviting and don't say anything about cost, people may assume that you will pay the cost of the meal and leave the tip.
I agree with Connie, however, since this lovely idea is yours, I think you must go beyond asking everyone to be available and requesting their time for a portrait. You should tell them before the action takes place, that you are happy to pay the photographer for services, but they are welcome to purchase negatives if they want to. Be up front about them purchasing the negatives, etc.
I suggest you present your question to the photographer who may be able to give you an idea of what to charge each member, should someone want buy a photo/negative for their own use.