SEO is the best way to go, affiliate with other game sites. Get a good staff on there and any friends that have a good knowledge of it. You can try paid posting but I only recommend a few for like 2 weeks max.
Pay to be on some high traffic sites/forums which get your site noticed. I would still first start out with getting a few active members instead of a ton of members that don't know what to post on.
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Forgot to add: contests another way, offer a prize