Membership alone can meet many of the needs of all of the mothers mentioned above. Try filling Group jobs with non-Leader helpers. Mothers who hold Group jobs feel a sense of ownership of the Group. They find support for their parenting, often making friends with other mothers in the Group who also hold Group jobs. Many Groups find that just offering a Group job to the mothers who come regularly will draw them into the core.
Different Groups handle offering mother-sized jobs in various ways.
Some make a general announcement of a job opening at the beginning or end of the meeting, others approach mothers individually about a certain job. Still others offer these jobs only to members or to Leader Applicants. However you choose to fill them, keeping these jobs filled is key to establishing and maintaining a core group. I think it's very important to note that paying attention to and spending some time developing the above two suggestions are very effective ways of developing a core group without holding any extra meetings. However, if you and your co-Leaders decide that holding extra meetings will best fill the needs of the mothers in your Group, then deciding which type of extra meeting will most benefit them should be your next move.