How can I create a subforum?
 

How can I create a sub-forum / forum using a php file?

I want to make it so a certain usergroup can create a hidden forum.

I don't know where to start!

Take a look at the forum table.

I am using this for one of my sites to integrate with my CMS.

You should be able to figure out most fields, most important being the options and the parent id. Once you insert a row in the table, you can call build_forum_permissions to rebuilt the permissions and genealogy (essentially population the childlist and parentlist fields)