The Disreputable History of Frankie Landau-Banks is a play on the history of the secret society that Frankie manages to find: The Disreputable History of the Loyal Order of the Basset Hounds. When Frankie discovers that book, she's still trying to get in with the Basset Hounds, even though she's a girl. She desperately wants to fit in and be accepted, and even thinks that if she masterminds the pranks and they find out, they'll initially be mad but might let her in after they've cooled off.
But over the course of the book, Frankie manages to make things happen all by herself. This isn't a story about Frankie as part of a collective; this is a story about Frankie. Just Frankie making things happen for herself and for others. The book therefore becomes Frankie's own story, rather than the story of the Loyal Order of the Basset Hounds.
That's why in the end the book is named just for Frankie and not for the whole group. She's alone in the end, having lost her boyfriend and having been rejected by the entire Loyal Order. But she's not defeated by it. No, Frankie is still going strong, proud of the disreputable history she's created all on her own.