Modify that anyone can participate in forum except admin

I am trying to integrate the punBB forum on my website.To access anything on the website member should be registered. So when member logs in then they could automatically participate in forum.But i tried to modify the codes and meet my requirements. I thought the changed codes could impact on other parts as well.There is already member table.How could I integrate that table with the user table created by the punBB forum. There will only one user i.e 'admin' who can start the forum categories and manage categories. Any help will be highly appreciated. Thank you