When I try boxing my 4 shammies with a Dk tank, I had the Dk specced frost. Now what i had for the assist macros was this:

Lightning bolt - Icy touch, single target threat is all that is needed so far.
Flame shock - plague strike, once again, single target threat.
Lave burst - (I use this early because i try to kill something almost straight away which helps with healing the tank) Pestilence, this will spread the diseases for for a small amount of aoe threat (relatively). The 2 first abilities plus white swings is enough threat to beat the aggro from lava burst. Also, I mash this button so pestilence goes off twice, more aoe threat and 2 death runes set up. Also, i have that talent in blood for 10% parry when bloods are on cd.
Chain lightning - howling blast, this is massive threat and at 80 will hit for like 2.5k easily non crit with decent dungeon gear. Easily enough to combat chain lightning aggro.

Rinse and repeat. That's roughly a 10 second rotation, if i need to heal I don't really bother with having any moves macrod for the dk while the shammies heal, makes it easier to keep track of the runes. you can do it differently ifd you want, but that's how i set up my dk tanking for my shammies.

PS: Also, if you death grip the first target, you will be up the the pesilence stage as the mobs reach you, which will draw them back to you if a shammy happens to pull aggro from mana totem (mana gains generate aggro).