Well try to get some of your friends to join up (besides your two friends you made it with), just in general people who could be interested in the theme as well.
Next to that make your board look as attractive as possible. Have a fitting skin, a nice layout. Not too much, but not to less boards. Fill up the boards with some topics, so when people join up they have immediatly someting to post in.
Try advertising on different boards, the more boards you advertise on, the more chance you have to get new members. Even if it gets you one new member, then your advertising has succeeded.
And as last, be different then others. Don't have board names that all the other boards out there have. Also you as admins should use proper grammar and spelling, and should be kind and active. Put a lot of time in it! Those are the only ways to make a succesfull board. And if people don't join up, then look over what you did wrong, try to make it better, or maybe think up a new concept?
Good luck
