Instead of doing those wonderful ways of making it active, I have a few good suggestions:
If it's only a forum and no main site then having a splashpage/static image (ie - A well worked on image that attracts readers in is a good way to make them stay on if they find the site.
If you want an active site/forum then in all reality, you need a domain name. Everyone seems to tyhink it's a big investment but for 10 bucks a year, it's not that bad.
Tell your friends about it and ask them to join and to tell others about it - always include a "picture link" to the site in forum signatures and then a desription about why it stands out from the rest.
And lastly, be unique, if you want an active forum then you need to have something to make it stand out. An idea completely thought from your mind - no copying or anything - that really is one of the most important things.
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Hope that helps a bit.