Unless you've experienced it first hand, you may not realize what the term "instance griefing" refers to. In this particular case, it refers to any player or players who are maliciously stealing your guild's raid IDs in order to grief you during raid time. (For non raiders in the audience, high-end raid dungeons have specific reset timers on them, preventing you from constantly farming the dungeon. At some point after zoning into a dungeon -- usually after a boss kill -- you're "saved" to the specific instance and assigned an instance ID that is the same for you and all of your group. Anyone saved with that specific ID will zone into your instance rather than a fresh instance -- at least until the dungeon resets.) Poster Trindade offers some advice on how random players might wind up getting your Karazhan instance ID:
Bob is a member of your raid and has your instance ID.
Jim is a scumbag griefer in shattrath wanting to steal your ID.
Ted is a scumbag griefer outside Karazhan waiting to steal your ID.
Jim is in a group with Ted. They have made their group a raid.
Jim whispers Bob "Hey Bob, wanna run Shattered Halls?"
Bob whispers Jim "sure".
Jim invites Bob to join his group.
Bob joins the raid group.
Bob is now the group leader.
Ted enters Karazhan.
So what happens next? Read on for the whole story.
Sure, Bob might have noticed something fishy about the group being a raid and himself being made leader, but then again, he might not notice these details at all. And when Jim and Ted make their excuses and drop the group, Bob might not even realize anything's happened. But now when Bob's raid zones into Karazhan, they may find people already there -- possibly people who delight in wiping your raid group for the fun of it. In the case of same-faction griefing (or cross-faction griefing on PvE realms) on outdoor raid encounters, Blizzard is usually willing to punish players who intentionally interfere in the encounter. After all, there's nothing your group can do if that level 50 decides to repeatedly run up to Emeriss and say hello -- but your group is going to end up wiping because of it.
However, in the case of instance griefing, Blizzard is simply telling players to never group outside their guild. Not that that's bad advice (I'm sure we all have our own special PUG nightmares), but never grouping with strangers seems a bit counter-productive to the idea of a "massively multiplayer"' game. However, if you're in a raiding guild and want to keep your instance IDs safe and sound, there's not much else you can do -- as Blizzard's refusal to act sends a loud message that this sort of griefing is a-okay.