Recently a change was made to prevent joining a party when the level was nearly done.
I'd like to make a suggestion on that: The default join should not be based on time-age of a level, but on completion. So far, every level I've seen has had an "all party" button around the half way mark. That looks like the best place to say "You can join before this, but not after this."
And more organized, but, there's one thing that needs to be implemented, to ensure no bugs: if the party is near the party switch (there are no more switchs to get to there, and no more rooms) the one entering would spawn at the party switch, instead of near the one he joined. This way, there's no chance of everyone going to a switch, someone with low conection joining, and slowing down everyone, because they would join near the leader, but not at the switch. also, would ensure that the one joining don't finish up stuck at the other side of the fence.