Welcome back! - The macro will auto-target enemies as needed, so essentially you just spam the macro. 
Having said that, it is good practice to apply Shadow Word: Pain / Purge the Wicked beforehand, which this macro (from my Shadow Priest set) can handle for you.
Theoretically, yes. And there is a Weakaura that will pop up if the buff is not applied to anyone in your group - to serve as a reminder.