Instructions
Recruiting Albel
Albel joins the party in Airglyph, after the first Vendeeni attack. He is appointed by his king to do so, and you have no choice but to accept his company.
While spending the night in Peterny, just before going to the Lava Caves, you are asked "Do you hate me?" Answer "No."
Before going to Moonbase for the first time, Albel will be one of the characters you can select to join you.
If you don't choose him for the initial trip to Moonbase, you can recruit him by beating him in a duel in the caves where you fought Crossel.
Recruiting Roger
Meet Roger in Duggus Forest, where he is trapped and requests your assistance.
Choose to free Roger, and he joins your party temporarily.
Later in the game, you run into Roger in Surferio. He asks for your help again, this time for the Seven Ancient Books quest.
When the quest is finished, you can make Roger a permanent member of your party just before you go to Moonbase for the first time.
If you do not choose Roger to go to Moonbase with you, he can be recruited in Surferio if Peppita Rossetti isn't in your party.
Recruitting Peppita Rossetti
Encounter Peppita in Hydra IV, just before the Vendeeni attack. She joins your party while trying to escape the attack.
Peppita later leaves the party and returns to her troop.
She appears again before your initial trip to Moonbase, at which time you can choose to bring her with you.
Recruiting Nel
Nel becomes a temporary part of your group in Airglyph, when she helps your break out of prison.
You have a chance to return the favor later in the game. She becomes captured at the Kirlsa Training facility, and you free her as part of the storyline.
You can choose to have her join your party just before leaving for Moonbase, as well.
If you don't take her to Moonbase, she can be found later at the Sacred City of Aquios. Start a conversation with her, and you will be given two possible answers to choose from. Choose the second one, and she will join you.