first part IS easy. lol. but i wouldnt do it.

here's why.

if your main is targeting a mob, you press the assist button so your priest will target it too, then cast its spell. well that works. sure. but if you need to switch targets, you'll have to press the assist button again. it works, but that's one extra button i dont wanna press.
instead, on my priest, i'd make my main the focus and always include, in my macros, that the first thing it should do it is attack what the focus is targetling. so if your main switches targets, your priest will never have to change focus.

in which case, do this when you make your party and forget about it for the rest of the day.
/focus party1

with your other spells, you will cast them like this:
/cast [target=focustarget] [] Smite

there are no options with cooldowns unless you use castrandom. in which case, i believe the macro would be:

/castrandom [target=focustarget] Mind Blast; [target=focustarget] Holy Fire; [target=focustarget] Smite
(note, since this is castrandom, it can cast smite even if it hasnt casted mindblast. just spam this button.

are you shadow or holy or disc?