I see that no one has posted here. I am still developing on how to get users to sign up and be active, at the moment I have a few that post around and add some content from time to time. I get sign ups but not the posting.
1. You have to have a clean looking professional forum. I think this goes a long way before the person even signs up.
2. Ease, It has to be easy to sign up.
3. Content or Offering, you have to give them a reason to sign up.
4. Maintain the community, rules, guidelines, enforce. Keep the community clean and on going.
5 Be active. If you the owner is not active nor will the people joining.