Members of the Association, identified by their email addresses, can access other information not available to the public. It's part of their basic member benefit. Some members who elect to subscribe to an additional service at a modest fee can access even more information, forms, and other benefits. They access that information by means of a password that comes to them as subscribers. With the advertising model, revenue is generated because advertisers want to reach the audience you're already reaching.
Advertising is basically an audience delivery service.
As an association, you can sell ads to people who might want to reach your members. You might offer reasonably priced ads as part of your service to members. That might take the form of setting up a Web site. Members could advertise there. The site could also contain a directory that would enable people looking for your members to reach them quickly and easily.