You need to configure IWT (Interact With Target).
And you need CTM (Click to Move) to be enabled.

You can find CTM in: Escape > Interface Options > Mouse Options > Check the box.
You can toggle CTM, via a macro:
(On) /console AutoInteract 1
(Off) /console AutoInteract 0

IWT is a Keybind: Escape > Keybinds > Targeting Functions



If CTM is enabled, and you push your IWT key, your toons will turn towards the target and move towards where the target was when you pushed the keybind.
You'll want to quickly give a movement key, such as W or S to break the movement.

If your software supports steps...
You could set it up like this:
Step 1: Assist the main character
Step 1: IWT Keybind sent to characters
Step 2: Movement key to break follow

If you're using IS Boxer, refine it a little.
Add... Execute a Step on Press or Release.
Add... Hold Keystrokes while Hotkey is Held.

That gets you...
Press (and Hold) Assist + IWT/CTM.
Release Break the IWT movement.



Any class that does not have a melee spec (your Shamans can be Enhance so they do) currently have a bug (or maybe new method of working that is not a bug), which prevents them from interacting (moving towards) a hostile character when they IWT.

If this is the case, instead of sending the IWT keybind to their windows, send a keybind for a macro/button.
And have a http://www.wowhead.com/item=41367 on that keybind or in that macro.