Have Mustadio in Chapter 4. After Bervenia, go to the mining town of Gollund. At the tavern, read the "Rash of Thefts" rumor. Go to the merchant city of Dorter. At the tavern, read the "A Call for Guards" rumor. When you leave the city a battle will start. Win the battle with Balthier's help and he will join you.
Beowulf
Have Mustadio in Chapter 4. Go to Goug. Read the rumor at Gollund. Go to Lesalia. Invite Beowulf to Gollund. He will join your group after a series of battles.
Cloud
Have Mustadio in Chapter 4. Go to Goug After Zeltennia. Return Reis to human form, then go back to Goug. Go to Sal Ghidos and win the battle.
Construct 8
Have Mustadio in Chapter 4. After rescuing Reis from Gollund, return to Goug. Construct 8 will join your party after an intermission sequence.
Dragon Reis
Have Mustadio in Chapter 4. Accompany Beowulf through Gollund. Reis will be rescued during the final battle in the series.
Human Reis
Have Mustadio in Chapter 4. Read the rumors at Zeltennia, buy a flower in Sal Ghidos, then go to Goug. Go to Nelveska Temple and win the battle. Reis will turn into human form afterwards.
Lip-Rouge
Keep Lavian and Alicia (Agrias' guards from Chapter 1) in your party in Chapter 4. Land on a City on Cancer I and have at least 500,000 gil. After the intermission sequence featuring Mustadio giving a birthday gift to Agrias, you will have the Lip-Rouge, which boosts the stats of female characters.
Loot
Make a point of picking up loot from fallen comrades and enemies. Crystals are very useful as these can restore health and magic power.
Luso
During Chapter 3, go to the Zekalus Desert after the Orbonne Monastary battles. After the intermission sequence, win the battle.
Sound Test
Start a New Game and enter 'PolkaPolka' (without quotes) as the main character's name. Make sure there are no trailing spaces and to respect the capital letters in the name.
The Byblos
Have Mustadio in Chapter 4. Complete all ten levels of Midlight's Deep.
Tutorial
If you are confused play through the tutorial, this will help answer most of the questions you have about the game.
Balthier...