I am in the process of getting my last team up badge in order to purchase the apartment. I spend a lot of time in teams, and I've noticed something: Too many players don't wait for everyone to be in the battle.
If you join a team up in a lower level world (Wizard City for example), odds are, at least one of the team members is actually questing. This means they need to complete whatever battle you've joined. Hitting with a critical tempest or meteor or blizzard or other AOE spell before the entire team is in the battle is incredibly rude.
We all know this, and yet, I've seen it so often. I've even asked others to wait, to no avail.
I have no idea if KI can even fix this. It's selfishness, plain and simple, and KI sadly cannot punish players for that. But if I'm in a team, I promise you I will wait, and ask others to do the same.
I think it might be a bit hard to find which people defeat the monsters before everyone is in the battle. I think some players just can be a bit impatient sometimes. Rarely, I team up with someone who will enter the battle while my character is in the loading screen.
If you're that high level that a single hit will wipe out the boss and his minions, you shouldn't enter such low level teams (WC & Krok). but if you do; waiting is courteous, let the low levels into the battle circle, you're not likely to be damaged by a rank1 2 boss.