Oh, and I would make it like this..

/cast [target=focus, nodead, exists, harm] Chain Lightning ; Chain Lightning

(You won't need the modifiers on your main toon, so the second on the list doesn't need them, the macro should only use the second part of the cast if it's the main toon.)

And if your characters are named toon1-toon5 and you have the change keys like F1-F5
Have F1 do a "/focus toon1" on toon 2-5 and have F1 do a "/clear focus" on toon1

Or switch castbars, but to be honest, this should work as well.