Outsource and pay members to play and compensate members for inviting new members. Write 3 gaming articles everyday with your gaming link at end and submit to your choice of article directory.
The best way is hire paid posters to start the ball rolling. After you have enough threads and posts, you can stop hiring them.
Its a classic catch 22 situation...people wont hang around unless it looks busy. So make it look busy by whatever means necessary ;-)