There are three guild quests you can undertake to raise the level cap for all your party members by 10 levels each: "The Dread Wyrm" (unlocked after "Call of the Wyvern"), "Awakening of the Serpent" (after "Remnants of an Age Past") and "The Azure Colossus" (after "Proof of Heroism"). Completing all three of these quests will allow your characters to reach the maximum level 99