I could tell you how to do it...but it would involve me posting a lot of the content of index.php, and even though it's not the whole file, the powers-that-be might not like it too much (and I wouldn't blame them). I would send it to you in a PM or email, but I have no way of confirming your license myself.
So I will tell you in general how to do it. I'm sorry I can't be more specific.
First of all, your homepage PHP file needs to have a statement that makes it require global.php. Look in your forums/index.php file, very near the top is a statement that says "require" and "global.php" in it. Copy that line and put it in the same place in your homepage PHP file.
Now look around in index.php. The major functions are identified fairly well. For example, you probably want the part that starts with "// if user is know, then welcome". Copy that entire section of code into your homepage PHP file. Look around for any other parts you might want, too, like the PM notification or Who's Online. All that stuff is in there. It's really not that hard to find, just don't be afraid to look.