Set up Click to move and interact with target in your WoW settings.

1. You should have some sort of /assist key to make your slaves target your main's target. I have a macro on [ that is simply /assist focus

2. Enable Click to move. It is under Interface > Mouse > Click to move. I set click to move ONLY on my slaves.

3. Assign a hotkey for Interact with Target. I use ] . You can set this under Keybindings > Targeting Functions {it is near the bottom of the Targeting Functions section}. Set this for slaves and master.

4. Target an NPC with your main, tell your slaves to assist you. Hit your Interact Key. The slaves will now move to the Target NPC and everyone will open the dialogue window.