Okay I've only tested this in Scarlet Monastery but I'm sure it works with other instances too. All you have to do, if you are the group leader, is set the dungeon on Heroic. Some instances don't have a Heroic version so when you try to enter the instnace, it flashes a loading screen but then says This Instance is not availble in Heroic. However, you are now behind the instance portal! This is fun in SM because a lot of alliance/horde people go there and will get freaked out to see you behind the instance portal.*Idea: Turn you PvP on and watch people try to kill you unless that person can range attack you but then you can back up .
Instances That Do Work With This Exploit
Ragefire Chasm(sort of)
Scarlet Monastery