This is a linguistic subtlety in the English version of Mailman.
Maliman refers to list subscribers as "members," for example when rejecting a post from a "non-member" to a "members-only" list. This terminology is confusing in some circumstances. Here's why.
I run lists for a professional, non-profit, organization which has dues-paying members. Not all paying members subscribe to all lists, and not all subscribers are paying members. If a dues-paying member has a posting rejected to what Mailman calls a "members-only" list, he will promptly complain to me that he is, in fact, a "member." And he's right. He's just not a *subscriber* to the list in question.
In my own Mailman 2.0.11 installation, I've scoured the sources for user-visible strings and changed "member" to "subscriber" everywhere I thought it was important. I would like to suggest this become an official terminology change for future versions of Mailman.