In Dragon Age 2, there are several optional party members that can join you throughout the game.
Isabela is a Rogue and does not join you automatically during the main quest line.
To get Isabela to join your party, head to The Hanged Man bar in Hightown after you’ve completed the Tranquility main quest. In the bar you’ll see a cutscene of her beating up some guys, and she’ll ask you for backup for a quest called Fools Rush In. Completing this quest will make Isabela join your party.
Note that during the quest, do not search any bodies until instructed to, or you may get a bug that will keep you from completing the quest and keep Isabela from joining your party.
