I used to use Party1, be we cannot control who the party leader is, in Battlegrounds.
If you will take the team into BG's, even just for a change of pace, I'd avoid the party1 if possible.
It won't work in the BG's, and I'd rather have a system that works everywhere.

Focus is decent, in arena you'd want it free for directing burst to one target without targeting them.
For general leveling, heroics and battlegrounds, you don't really need focus.
And that's the easiest system to base your macros off of.

Whatever software you are boxing with, you have PiP swap hotkeys.
Create one warcraft macro for each toon on your team.
So a five member team, will have five macros on each team member.
Assign the same keybind in warcraft for a toon as the keybind to PiP swap to that toon.
So when you PiP to toon 2, everyone runs the macro for Toon 2.

Your macros are to the effect of: /Focus [Target=Toon2]

Then you create a follow macro for all toons, on the same keybind.
It will be something like /follow [target=Focus]

For hunters and melee toons, your attacks will be:
/assist [target=Focus]
/cast Sinister Strike..... or whatever.

For everyone else (casters), your attacks are:
/cast [target=FocusTarget] Fireball..... or whatever.

And then heals use double conditions:
/assist [target=Focus]
/cast [help] Renew; [help, target=TargetTarget] Renew

If your leader is targeting a friendly, the heal goes to the target.
But if the target is hostile, it goes to the target's target (which is almost always your tank).