5? Including yourself you have four... not counting the creatures you can summon. You can have tons to choose from, and they will reside at your party camp.
Try to update the game to the latest version, and it will get a bit easier.
Mages are quite weak early on, but later become easily the most powerful. I wouldn't spend much on constitution for the mage... You keep the mage at the back covered by the two tanks (Alistar and Sten).
If you have the stone prisoner DLC, I'd go there first. The NPC you unlock there is the best tank in the game, and can easily take the most damage.
So right, keep your mage and your ranged attacker (the bard chick) at the back and the two tanks upfront.
With your mage, the magic points determine the spell power and the willpower affects the mana level.
With Alistar concentrate on filling the sword and shield skill points, and invest in dexterity, and strength... and some in constitution. Make sure you give him a one-handed weapon and a shield of course.
With Sten, concentrate on the two handed sword skill and invest in strength and some constitution. Give him a big sword two handed weapon which blocks out the shield slot.
If you do the stone prisoner DLC, the NPC there can take tons of damage. Just concentrate on his constitution and strength.
I wouldn't auto level the other characters. To me, it is all part of the fun.
edit:
Another piece of advise I have is that you give each character an alternative ranged weapon. You can switch weapons with a single hotkey.
Later when your mage is casting blizzard spells and firestorm spells, that can also hurt your party, you can have them all switch to range so they hold back, while you cast a big spell like earthquake, blizzard or firestorm.
Aside from the stone prisoner, another great NPC is a mage you get from the mage's tower quest. That is EASILY the best healer in the game, and she really makes the game significantly easier.