Complete the indicated task to get the corresponding person into your party.

Person in party	How to unlock
Blackwall	Found in the Hinterlands after Leliana gives you "The Lone Warden" quest.
Cole	Appears later in Haven and is obtained as part of the storyline.
Dorian	Found in Redcliffe after Fiona requests for the Herald to meet with a faction of Mages.
Iron Bull	Wait at the gate doors outside the Chantry in Haven until a messenger appears to invite the Inquisitor to the Storm Coast.
Sera	Enter Val Royeaux, the capitol of Orlais and notice the red glowing arrow near your Inquisitor. Find all the arrow messages around that city.
Vivienne	At the opening courtyard of Val Royeaux, search for a messenger near the fountain on your way out of the city. You will receive an invitation to meet with Vivienne at her chateau.