From time to time you may want to create a forum that not all members want to be part of; so you make it opt-in this way those that want to see and participate can opt-in. The members that don't want to see the forum won't have to do anything. For instance, we do this with a politics discussion board on my forum. Create a new user group based off either registered users or your promoted users group. Make it publicly joinable by setting "Public (Joinable) Custom Usergroups" to Yes. Next create the forum and set all permissions to No for all member usergroups except mods, super mods, admins, and the user group you just created. Now your members can join the forum right from their User CP under "Permission Groups". Upon joining a user will be assigned to the secondary group.