- If I was to pick an external player I'd almost always go for a healer. Because if it's a tank you'll need to heal him as well yourself, and my experience is that healing an external player adds more complexity as you have no idea about his cooldowns or the moves he is going to make. Whereas an external healer will take a huge amount of work out of your hands.

- You could spec your druid into a tank. However it's not really needed for marrowgar if you are geared, use cooldowns, have enough healing, taking into account the 30% buff.

- If you don't want to spec the other resto druid into balance, you could always convert your priest into a shadow buff. Misery gives the same hit buff as improved ff.

So if i was you i think i'd go for (taking into account the 1st 4 bosses in icc).

- pally tank + druid tank
- druid healer
- 3x ele shaman
- mage
- shadow priest with disc off spec
- resto shaman with ele off spec

depending on the fight go with 2 or 3 healers, and this will also give you the possibility to clear bosses on your own if your mate can't make it.

If he brings over his druid he can depending on the fight go for healer or dps (and as a dps he still has self healing abilities for when you are in trouble).

If you clear the first 4 bosses in icc you'll get many decent upgrades. Along with running some hc dungeons and you'll have a very decent pve spec in no time.