Getting forum users can be quite tricky.
You have to remember that people would visit your forum but they're not going to participate unless there is consistent and constant activity on your forum.
If I were you, I would hire two virtual assistants.
One VA is going to focus on simply keeping your forum active.
Give this person several dozen accounts and how that VA post all sorts of stuff to make it look like the forum is active and has many participants.
Also, make sure that all forum posts have a proper SEO.
What I mean by this is that there as to be niche-specific keywords in the forum post title.
What makes this tricky is that the forum post must be useful.
So, the best approach for this is to get the VA to look at question-and-answer forums or question-and-answer resources like Yahoo Answers and get ideas there for forum discussions.
Also, stir up fake drama among different fake forum personalities.
Try to make it look real.
The other VA should drum up traffic to your forum.
Since there's a lot of questions being asked at your forum and a lot of answers are being supplied, the promoter VA should go to Facebook groups, Facebook fan pages, Yahoo Answers and other resources and try to drum up traffic that way.
Also, other forums are great places to get traffic as well.
It's also a good idea to post your forum discussion links on related blogs.
Don't expect your forum to gain traction overnight.
It's going to take quite some time but if you're able to grow an inorganic community and it becomes self-sustaining, the amount of effort you need to put into your forum can decline very rapidly.