Migrate them from a similar forum, e.g. a lot of boards.ie - webmaster forum users are members here. It wasn't that the users were robbed from boards, but I came across links to discussions here from boards. Nothing unethical about it.
Don't go advertising your site on another forum but use that forum and create a sig with a link in it. It'll take time to build up but slowly but surely you'll get your visitors. Just be ethical about how you get your users!
