Known issue with players entering a map that is mostly complete
The team is working on an issue that is allowing new players to enter a map that has already been started in earnest. You may have experienced this problem in action by either entering a map that is partly or mostly complete, or by being most of the way through a map and having someone join at that point.
The intended way for map joining to work is that it won't allow people in after a certain threshold of monsters have been defeated. Look for a correction to this issue in a future patch.
