Group leaders should always mark targets so that the party is on the right page...
Most common markings are SKULL (1st kill), X (2nd kill), MOON (3rd kill) CIRCLE (4th kill). 3rd and fourth kills should be sapped, sheeped, banished, enslave, etc. for immobilization.
Tank is always first to attack SKULL and the rest of the party members should ATTACK THE SAME TARGET after he attacks. The tank will pull aggro and they should focus on him. Than on to the X, MOON, CIRCLE, etc. This also makes it easier for the healer to focus on healing one or two guys and not the whole group. Alot of times the healer will pull aggro, thats when a pet, off tank or spell should get the aggro from healer. If tank catches all the aggro like it should, it should'nt be chaos and all the creatures should stay in a group. *Keeping them in a group is the best way to clear.
If targets are not marked when running an instance, theres gonna be chaos cause everyone might not be on the same page and they'll start attacking other targets. It also saves time typing on which to kill 1st, 2nd, etc.
*TANK HINT- When tanking, you should pull the enemy group away from the target that has been sheeped, sapped, etc. If you have them all in a group and attack while 1 is sheeped, sapped, etc. theres a good chance it will effect the immobilization. Its also good to pull group away from the one that is immobized so the party can AoE.