Internal groups in Set/I

Question

I have a question. Consider the category $\mathbf{Sets}/I$ sliced over a set $I$. Now we can think of this a group in a category, right? Now we have to show that such a group $G$ determines an $I$-indexed family of groups $G_i$ by setting $G_i=G^{-1}(i)$ for each $i\in I$. Any tips or hint are appreciated.