Well... As a user only becomes a member of your board when they post, nobody would ever be able to get into your board, as they can't become a member.
what you can do is to either use Member screening ("Edit Board Settings"), or make the forums private to specified users, mods & admins, then just specify that all your users are allowed access.
For the Member screening option, thet'd stop people who aren't members of your board from viewing. They click a button to get your approval, then you either accept or reject them. If you accept them, they become a member on your board, and can see/post.
For the private option, to specify that users can access, click on "Edit Users", then check the members you want to have access, and choose "Set Private Forum Access" from the drop-down menu, and select the forums.