I have my alts /focus my main and then use focus in their targeting so I don't have to /target ever, ever on my alts. My attacks are
/cast [target=focus-target] DPSspell
and heals are
/cast [target=focus-target-target] Healspell
So my main selects a mob and then I push the key that is bound to my attack macro, and bam, all the alts attack what my main is attacking. And I press the heal spell, and without knowing who has aggro, no targeting people, no changing targets from the mobs, no worries, just healing goes to whoever has aggro.

Now, on my hunters, I have a few attacks, but some are just shoot, send pets and shoot, full rotation, send pets and full rotation, just send pets and just pull pets back. Some things might change based on the "new" defensive mode of pets, but you get the idea. For my times in instances, I split up the damage my pets take by putting a mob on each pet. I select a mob for each hunter and then push (send pet and then attack). For that I just use a simple /petattack, which the pet attacks whatever the char has targeted.

The part where the alts assist the main looks like:
#showtooltip
/stopmacro [target=focus-target, nodead, noexists]
/petattack [target=focus-target]
/cast [target=focus-target, combat] !Auto Shot

You don't need the first part. It says, don't do anything if the main has a dead mob targeted or if he has no mob targeted. The second part sends the pet at whatever mob(target) the main(focus) has targeted. The third part is to attack the mob(target) that the main(focus) is attacking, but don't do anything until the alt is in combat. Which doesn't happen until the pet gets to the mob. That way, the pet has a chance to get aggro first.

Hope that makes sense and helps.