Dear Ben,
you may also find the following topic raised by @are_ess helpfull:
Q&A: An absolute beginners question:
Under this topic a number of suggestions for readings were given, which would very likely fit the purpose of your study group. Other than that, I agree with @LXNDR that Bhikkhu Bodhi’s “In the Buddha’s Words” is one of the best and could be studied in your group in the way he describes. I also found this book immensely helpful myself.
With much mettā,
Robert