For more engagement specifically locally, there are a couple of things that come to mind. Share it on your Facebook page to get it out there, you can also link the group in various places such as email signature and blog posts where relevant. You could also invite your Facebook friends to join the group, getting the word out there to your target audience is the key.
Of course in general posting regularly in the group and creating fun and engaging content will help participation… I know if I enjoy a group I’m in, I’m more likely to tell a FB friend about it and link it to them.