A one-box shammy will level the fastest Enhancement.
A five-box, or really any multiple box, I'd go Elemental.

Down the road, you have a choice between respec (or dual-spec) one to Restoration.
If your group is 4x Shaman + 1x Tank, the Resto dual-spec can make heroics so much easier.

If your group is 5x Shaman, all Elemental will be better then Resto + Elemental.
I don't see 5 shammies doing heroics without a tank, until you outgear them.

Elemental has a pretty basic rotation for PvE.

Once you're 75, its Flame Shock, Lava Burst, some Lightning spells, another Lava Burst.
Recast Flame Shock once it expires.
Essential glyphs would be Flame Shock (Lava Burst does not consume it), Lava (10% more spell power to Lava Burst) and I'd go with Totem of Wrath, but last can be whatever.

Until 75, the rotation is:
Shock of choice (Frost - snare, Flame - highest overall damage, Earth - largest single DD shock), Lightning Bolt spam.

Chain Lightning can go with Lightning spam, anytime there's no CC (sheep, sapped mob, shackled undead etc).
It's less efficient but harder hitting, and will jump to 3 additional nearby targets, less damage per jump.
In elemental you have a talent which reduces the mana cost of spells after a crit.
Since Lava Burst automatically crits if Flame Shock is up, its good to Chain Lightning right after Lava Burst.

You have a few others spells of note.

Heroism is a massive raid/party wide dps boost.
Hex is a really weak version of crowd control.
Purge can get rid of a bunch of the opponents buffs.
Wind Shock (like Earth Shock) can interrupt casting.
Lesser Healing Wave is a fast cast, think Flash of Light (Pally) or Flash Heal (Priest).
Healing Wave is a slower cast, bigger heal, think Holy Light (Pally) or Greater Heal (Priest).
Chain Heal is good anytime your whole team needs heals, if 3+ need a small heal its a good choice to use.

The other big thing of shammies is totems.

Each shammy on your team can drop one totem from each element.
Pick the best totems of each type.
Mana won't stack with itself, so you'll only want one.
I might go with:

Fire: Totem of Wrath, Frost Resistance Totem, Searing Totem, Searing Totem.
Water: Mana Spring Totem, Cleansing Totem, Healing Stream Totem, Healing Stream Totem.
Air: Wrath of Air Totem, Windfury Totem (only if you have a melee tank), Grounding Totem, Grounding Totem.
Earth: Tremor Totem, Strength of Earth Totem (again, only if you have a tank), Earthbind Totem, Stoneskin Totem.

Make this into a big macro for your shammies like this:
Just move the spells around, so each shammy has one from each element.
You might drop essential totems first, so keypress 1 drops Totem of Wrath, Tremor Totem, Grounding Totem, Mana Spring Totem, etc.


#show
/castsequence reset=5 Totem of Wrath, Mana Spring Totem, Wrath of Air Totem, Tremor Totem


Some totems you'd like the ability to drop individually instead of part of the big totem macro.
Even though you have 2 or 3 groundings in your totem drop macro, having all shammies drop one on a press can really help vs casters.
Dropping all Magma Totems can help with AoE (or 3x Magma + 1x Totem of Wrath).

Other totems you'll want on a castsequence.
The spell name is a cast, the comma is a null where nothing happens for that shammy.
Some totems which will benefit from the sequence include Earth Bind (for the root once talented), Fire Nova (for the stun once talented), Fire Elemental, Earth Elemental.
Thunderstorm will be a good spell (not totem) to round-robin once you get it (talent obtained)

/castsequence reset=12 Fire Nova Totem,,,
/castsequence reset=12 ,Fire Nova Totem,,
/castsequence reset=12 ,,Fire Nova Totem,
/castsequence reset=12 ,,,Fire Nova Totem

Give each shammy the macro once, on the same keybind and you're set.

Hopefully that gives you a few ideas for shammies.