on each character, make the following wow macros:

-------
# focus macros, name in parenthesis
#
macro(1): /focus joe
macro(2): /focus sam
# ...continue for all characters

# standard assist and follow macros
#
macro(assist): /assist focus
macro(attack): /startattack [target=focustarget]
macro(follow): /stopattack

/follow focus


# now a common targeted spell macro
#
macro(fireball): /cast [target=focustarget] fireball
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assign them to your in-game keys however you do it (i use TrintyBars)

group up... hit the focus key once... then you are ready to go. each character will now use the focus for targeting.
just click a target (or tab), and fire. rinse / repeat.