There isn't really much content there to attract new visitors. I realise it's a forum and the vast majority of content is generated by the users but there needs to be a reason for people to join and at the moment there just isn't.
You could try starting a few discussion threads, asking for people's opinions, suggesting your own hints and tips for different game modes.
Don't try and get members by saying something like "the next 5 people to register with be allowed to be moderators." I've seen a few people try this in the past and it just makes you look daft.
You would probably have more luck creating a site first and then basing your forum on that. Perhaps a blog dedicated to gathering all the news and rumours about the latest Call of Duty game? You could include write-ups of professional Call of Duty matches or some longer articles about the history of the game itself, I remember playing the original on the PC and I'm sure there's the possibility for an article about how it has influenced modern day first person shooters. From there you could encourage people to discuss the points further either through comments or in the forum.
If you do decide to try this then don't show it to other people until you have a good amount of content there. First impressions count for a lot on the web and if there is nothing on your site the first time people visit then it is unlikely they will return.
Good luck.
