Given that everybody can know the same stuff its hard to assign roles. Still, I tried.

Vaan ~ Attacker with a short sword. Occasionally I use him as a thief, but I get annoyed switching his gambits over when I want him to attack.
Balthier ~ Attacker with a spear + support.
Fran ~ Archer + support.
Basch ~ Attacker with a ninja sword.
Ashe ~ Attacker with a dagger.
Penelo ~ A mage.

Though, as I said, they all know enough magic to deal with uncertainties and to heal each other quite well should something untoward happen.