You recruit Ronny at the beginning in the sewers (it's kinda forced), Sala is recruited by unlocking his cell in the cult's mansion with the keys you get from warden, Warden is recruited after you defeat him (tipping over his chair starts the fight) and do all the following quests he asks for in the mansion (remember to follow him, when he tells you not to), after you exit the mansion he should join your party. Interrobang joins you after you get knocked out by the thieves. Now, lastly, you recruit Ari Aida in the thieves' prison, you have to unlock her cell with the keys you obtain after defeating Varek and then she joins you.