there are many ways to do that. some of the more popular are setting up your macro's like this.

/cast [target=focustarget] spell - (Setting up your main as focus on all accounts)

/cast [target=party1target] spell - (with your main as party leader)

/cast [target=MainNametarget] spell - (not as flexable but it works)

/cast [target=targettarget] spell - (have your other characters targeting your main)