Can you ask the people in the group to recommend it to people they think might be interested? I believe this is called "snowballing." Maybe have a night where each person needs to bring a guest?

The blog might help at promotion if it were about the group. It would at least be something you could refer people to who wanted to learn more.

Can you list your group at websites in your area that list groups?

Does your area have these free magazines you can pick up on street corners and libraries that list all sorts of classes for adults? Like "Discover U" or similar? If so, consider offering a course through them (people pay a nominal fee to participate), and make it have the same format as your group. Meet for a few times, and then the course is over. You can funnel any people from the course who liked it into your self-help group. Repeat as necessary.