Use

/assist focus
/cast Spell/ability

or

/cast [target=focustarget] Spell/ability

I use the former so I can see when they have acquired my target.The second one sucks (imo). Why? If your main dies or you want to attack something with a character who is not in range of the main, it will NEVER attack because it must have that target to attack. It's annoying.

I don't use nodead or harm modifier because it won't cast the spell if it can't cast it anyway. All it does is prevent error message spam which doesn't really matter anyway.

Keep in mind no matter what you do there will be targeting delay. Your latency times two is the delay between when you target something and your followers can pick it up.

Additionally, you may find this macro useful for target acquisition:

/assist focus
/startattack
/follow focus
/cast !Shoot

That will automatically acquire a target on your main and everyone else if don't have one (it will pick the closest enemy in range). Generally this is helpful as it avoids the /assist delay. There is a chance your followers will target a different mob than you do at first, but after the latency delay will pick your target if you press it again. I generally use it as my normal /follow button to spam and to make sure everyone has the target acquired. Melee will autoswing, hunters will autoshoot, and casters will wand if standing still.