The title of The Reformed Vampire Support Group combines a lot of neat ideas that don't normally go together.
If you've ever watched Buffy or Angel, then yeah, you're familiar with the idea of a reformed vampire, a.k.a. one that doesn't go around senselessly killing all the time. So that makes sense. But we've never heard of a vampire support group before, nor of a support group specifically for reformed vampires. So right from the get-go, the book's title is suggesting that reformed vamps are a thing, and that they might need a support group.
This all makes sense once we learn about Nina and her gang: They're trying hard to be good non-lethal vamps—really, they are—and the support group helps them with this goal. It also helps them band together to summon the will and find the resources to stick up for themselves once a vampire slayer is on the loose. Strength in numbers definitely seems to apply to this situation, so it's a good thing these guys were already in a group together.