As a thought to take on board, I had an idea that could bring in a vast majority of users.
The idea stems from new game releases (mainly PC) and the BETA keys that are sent out to websites that work in association with the release.
With the gaming market being a big hit for small development companies managing to release FTP (free-to-play) games online, the ones that are in development tend to hand out a certain amount of closed beta keys for eager gamers to get their hands on.
If you was to contact these websites and offer advertisement links and article space in the news feed for up and coming games, i would imagine that they would be more than happy to send a bunch of keys for you to then hand out as prizes in draws for new and existing members. you could also run a competition for current members to recruit a certain amount of members, with the eventual prize of said keys or higher site ranks.
social media sites, such as Facebook, twitter, google+ would be great places to advertise these competitions.
having seen websites swell firsthand from beta key competitions run via Facebook and twitter, i can attest to its success.
other ideas that come to mind are sections for Twitch users, Twitch is a website that lets its members host real time gameplay and commentating. This site pulls in a huge fanbase and would be a great asset to be associated with.
i will post anything else i may find useful, i hope this helps.