I am thinking that pretty much through a mesh network of trading that everyone will eventually have everyone elses spells. If there is just one popular person in the group that everyone trades with then that person will have collected all of the spells that everyone has, and since that person trades with everyone, then everyone will have everyone elses spells.
We can save time by just skipping the intermediate steps since we already know the basic outcome - everyone will have access to everyones spells.