How to create
 

How can We create a cover page for one of our sites that a person must click on I agree before they can enter the site.. ?

if your site is setup as /forum its easier because the default page of public_html is index so any page named index.php or index.html will be default. then it would control your link of acknowledgment. you would then change your cms page and forum home in setting/option/Site Name / URL / Contact Details so they stay in /forum this will force the link to be the only way in unless they go around the front page, but by doing this circumnavigation they would be knowingly bypassing the acknowledgment page,
And that would still cover you as far as any agreement, you are trying to convey

I have a portal page and the forums are off of the nav-bar

ok so just replace the portal page with the new index.htm and direct the link to your portal page i guess