try to add $1 for signup to increase your users and continue the pay till get more active users..
investing is the only key to get more traffic for your new forum.
Thank you for your share. We will consider adding signup bonuses in the future!
Sign up bonuses will be so easy to be abused, so be careful before implementing it if you want to do it. At least prepare a great system to avoid abuse.
One suggestion, maybe you need to make a contest for the best poster weekly or monthly with interesting reward.