well i took my warlock out to do some world questing in outland to take a break from boxing for a bit and came across an ally DK who helped me kill a mob. wonderful. i figured i can help them back... so i tried and ended up attacking the DK. lol. killed him when i saw his goul attacking me. only after i killed him did i realize my macros dont work if im helping an enemy player. here goes...

/cast [target=focustarget, harm] [target=targettarget, harm] [] Corruption


so i didnt have a focus, but you can see that the second part of my macro will target the ally. =/
i guess i wont be helping any allies from now on.