Set your focus to party1:

Code:
/focus party1
Target whoever your focus is targetting:

Code:
/assist focus
Cast a spell on whoever your focus is targetting:

Code:
/assist focus
/cast Whatever
or

Code:
/cast [target=focustarget] Whatever
Follow your focus:

Code:
/follow focus
You could bind different keys to /focus party1, /focus party2, /focus party3, etc to allow you to quickly switch focus to a different member of the party. If you've set up all of your spell casting macros to either assist the focus or cast at the focustarget, and your follow macro is set to follow your focus, everything will just automatically start working when you switch focus. You'll just have to start controlling whichever character you switched the focus to.

I hope this makes sense...