Joy says, "i think our reading group reads more sophisticated literature than jenna's group. frankly, between you and me, it's because most of us have a graduate degree." in using the word "we," joy is referring to her:

Joy is referring to her in-group.
An in-group is a group of people sharing particular characteristics. In the case above, those characteristics are graduate degrees. Joy belongs in this group based on the fact that she too has a graduate degree as well as the rest of the members, which is why she believes they should be reading more sophisticated literature than the other group who don't have degrees.