When you think of how you will present membership, I suggest you focus on the benefits of membership to the mother. When a mother becomes a member of LLLI, she becomes one of the group. Most importantly, she is more apt to breastfeed longer. A Leader is better motivated. Point out that the Group and LLLI are supported by memberships.
Many mothers nowadays are no longer familiar with nonprofit groups.
You may want to mention specific examples of how LLL and you as a Leader help mothers and babies. Highlight new library books, etc., bought with membership funds. At the end of the meeting, remind mothers again how they can become members. In doing so, you show that you appreciate her joining the Group, as well as let the attendees know that other mothers are purchasing memberships. Have a space to indicate interest in membership in the sign-in sheet.