Well, that's simple enough. Once Banks had one Angel in mind, she just worked to convince Kristen Stewart that the film was one she should be a part of. Aside from being the group leader, the only other thing we know about Sabina is that she's good at partying. How that will help in her work as an Angel, I don't know, but Stewart has taken on a wide variety of intriguing roles since her time with the sparkly vamps of her youth, so there's no doubt that the role, and film as a whole appealed to her, and that she'll bring something cool to her work as Sabina.