You've got two choices, with proponents on each side.

If its Paladin plus 4 Shammies, you can either go:

a) 3x Elemental and 1x Resto; or
b) 4x Elemental

With A, you can have dedicated healing and constant DPS.
I've gone this route, and would recommend a macro or hotkey to target each individual.
Then for your main heals, set them up so you assist the tank for each of your heals.
I use:

#show Riptide
/assist Tank
/cast [help] Riptide; [help, target=targettarget] Riptide; [target=player] Riptide

You can change the last [target=player] to target=Tank or whatever.

I really like having Mana Tide and Earth Shield for more difficult content.





With B, you have even more firepower when you want to kill things.
However, when you need heals, there is no dps.

Both methods work, its basically a preference.