Szczecin looks to be the nearest airport. I think you really need to be confident about your numbers - there will be more choice at around 5 people than there will be at 15, but if it is going to be up at that sort of number Loganair would be a good starting point given your preferred departure points.
Another thing to consider is what will your baggage requirements be e.g. will you also need to transport bulky/heavy sporting equipment