On one email list I moderate, I tell all participants in an automated welcome message that they are encouraged to introduce themselves to the list, and that once they have, they will be taken off the moderation of submitted messages that is the default on that list. Over time (four years), about a third of the subscribers have posted, as I can tell from how many users are no longer on moderation. There are almost 500 subscribers on that list, all sharing a membership in a state organization of which I am president.