Disclaimer: Characters are not mine.
Cosette is introduced to the Independents by Marius, and when she first arrives, she's overwhelmed. They're such a tight-knit group, and they debate about…well, everything, and not just polite arguing, but finger-in-your-face, voices-rising, enjolras-broke-a-glass-at-the-second-meeting-she-went-to-because-he-was-so-angry debating. The first few meetings, she remains quiet, if not silent; however, by the fifth or sixth meeting, they're all enamored of her.
She teaches Jehan how to French braid, and he soon begins greeting her, as he greets everyone else, with a kiss on the lips. She sneaks Courfeyrac out of statistics class and takes him to get tacos, and he starts laying his head in her lap at meetings. She takes Grantaire home when he's too incoherent to find his bike, and he starts putting his arm around her when she sits next to him at meetings. She argues that there's "no wrong way to be a woman" when a scandal involving a female senator denouncing another for being too feminine emerges, and Combeferre gives her a nod of approval. She picks up after Bossuet when he breaks something, earning his love, as well as the love of Joly and Musichetta.
The only two who don't immediately adopt her as one of their own are Enjolras and Eponine, and, though it bothers her a bit, she's much too happy with her wonderful freckled boyfriend and his insane motley crew to dwell too much on it. She's melded into the group, and it's as if she's always been there. She can't remember being happier.