1. When you reach Chapter 4, head over to The Mining Town of Gollund.
2. Check the "Rash of Thefts" rumor from the tavern.
3. Drop by The Merchant City of Dorter's tavern and read the "A Call for Guards" rumor.
4. Leave town, and you'll automatically enter a battle.
5. Win with the help of Balthier, and he'll join the party.