Most group content in WOW is tightly regulated: five-mans are tuned for ... errr ... five people; raids are designed for ten or twenty-five. If you've got 11 people available for raiding tonight, someone has to miss out. You can't just wander up to an instance and help out, either - if your name's not on the list, you're not coming in.
The July losses of the minions of Evil
5 hours ago