a brief discussion about the site you want to patronize and a link to where to have the membership. that would be the simplest way. thanks

You can put its information on other related forums,and you can answer other people's question with your links below,I believe it can attract some people to your forum.
Good luck.:D

You can follow some useful tricks to get members on the forum. If you have some good members who really want to stay then, it is beneficial for that forum.