I had an issue like this with my shaman team. It was the last step just before you "empower your artifact" you become part of the earthen ring..after that little cerimoney you go talk to the nps infront of the cave that leads insdie the class hall but the entrence is blocked..you click on the rocks to clear them and then follow the npc inside. When i clicked on the rocks they cleared for everyone but only my main toon could see and follow the quest npc inside.

I had to abandon the quest on my alts.. port them back to dalaran, when they came back the quest was reset. SO it does seem to be an "in a party" situation. At some points in the class quests, if your grouped and in the same instance/area..if one member clicks on somthing to complete a step..it bugs the rest of the party.

Reading what you guys are talking about, it might be good to have your team on follow but not grouped for class quests? So this is not a multiboxing issue.. its a doing class quests with others of the same class grouped with you and on same step ( yes it effects us..but would also affect anyone grouped like this doing class quests together )