Register 4/5 users yourself and start threads related to the forum topics, then respond with another of the fake accounts.

Stupid, but it will give you content to begin with

One thing I would suggest is changing the skin (style) of the forum.

There are buckets of forums out there that look exactly the same as yours, and people won't stop to read what your forum is about.

You need to plaster it into them what they would be doing on your site, and why they should stay there to do their bitching.