NIN/WAR (tank)
RDM/WHM or /BLM (healer, debuff, dmg)
BLM/WHM (dmg dealer)
MNK/WAR (dmg)
BRD/WHM (buffs, healer)

Sixth member could be WAR/NIN or RNG/NIN.

Note: I quit XI as the new jobs were added, so I don't know too much about how they fit in the PT setup.