I would like to improve on criticalss's list.
First of all-
Cleric should be first priority.
Warrior should be second, although rogues can aggro and tank really well. They make a temporary subsitute.
Wizards should be third.
Fill the rest with whatever you can get.
A good party looks like-
2-3 Clerics, atleast one with Bard submastery.
Two tanks, preferably both dual axe. 1h and 2h warriors keep interupting spells and aggroing more mobs with knockback!
One or two wizards, preferably one wizard and one warlock.
The rest is open for whatever you can get, preferably a rogue or another wizard.
Roles-
Warrior- Take all the damage, deal out a good bit.
Advice- Try to avoid knockdowns as much as possible, wizards will come to hate you for these.
Cleric- Heal heal heal! If you see a party members health drop even slightly, HEAL. Do not stop for anything aside from dying.
Advice- Keep your party buffed.
Wizard- Finish mobs quickly. These guys are DEFINENTLY some powerful beings.
Advice- Start out with your weakest, non aoe nuke (ice) and about halfway through just unload with your strongest nukes. Use Life Control only away from AOE mobs.
Warlocks- Debuff, and damage over time. Always open with your debuffs, then pull out some damage over time.
Rogues- Damage dealers, good tanks.
Advice- If you have to substitute for tanking, do NOT use desperate dagger/extreme crossbow. These will surely get you killed.
Bard- Just keep mana replenishing, the chance you'll have a bard at all is slim, usually clerics will just invest some sp to get the speed buff.