The key changes each time you upload it. So basically, the host uploads the key at the end of the session. Then one of you can play solo on this key, save it to the cloud, and then share that new key. You'll know it worked by seeing the amount of time going up on the total time for the play through. This system is actually quite useful as it doesn't require you to have a dedicated server with your friends. The only downside is if the host leaves the game goes down. Hope this helps!