When it comes to choosing a bible study curriculum there are a lot different kinds that you can choose from. which one you choose has a lot to do with the needs and makeup of your group. but one particular style that I like is a book study. This kind is where you and your group take a book of the bible and just walk through it piece by piece.
Like with any bible study there are a few things that you want to be aware of. You want to make sure that the author is credible. You want to make sure that they are a good writer but more importantly that they have solid beliefs and theology. often this can be found out by doing some quick background research online and through informal questioning with those that you trust. You also want to make sure that you choose a bible study curriculum that covers a book that works well for your group.
While each book has beautiful truth to share there may be some truths that are more pertinent to your group. for example, if they are struggling with the basics of their faith you may want to start with one of the Gospels or Romans. if they are struggling with trials you may want to choose a book like James. You know your group though and that knowledge combined with prayer will direct you to what God would have you to study.
You may be wondering why I prefer a study such as this. one reason is that you really dig into scripture. Through doing this your group learns the truth the bible contains and how to study in such a way that they can uncover this truth on their own without having to have a bible study curriculum. not to mention that this type of study does a great job of doing expository study along with topical application.
The type of study you choose is up to you but I think that a book study is a great place to start. It gives your group a strong footing in how to study scripture so that they can learn how to grow spiritually on their own at some point. Because after all we should not just be feeding people fish but we should in fact be about teaching them how to fish. that attitude is what will make your study group effective.