At least hot water and fixings for coffee, tea, and cocoa. Eat the food at the break halfway through. This allows members some valuable socializing time, which in turn helps them to feel more comfortable in the group. Coordinate food with the setting of the book. Most libraries do not require pre-registration for book discussion.
New members are welcome at any time.
Only reason to pre-register would be to predict how much food is needed. The participants vary each month. You will probably attract very few men to your discussion group. Books and articles discuss at length why book groups tend to be predominantly females. See Jacobsohn, Laskin, and Saal books for guidance about what a book discussion leader should do and the qualities one should possess.