Personally, I don't think a set method is the way to go, but having multiple ways to get involved. For some elaboration, Nymeria and I have a general storyline in mind that we would like to guide you through. That's going to involve a lot of group missions, where everyone get's involved, where as if players just want to go on a hunt on their own/with a small group then that option still remains entirely open with the bounty board. Speaking to @Migyudon's idea, the general party size based on what the hunt is makes complete sense to me.