Well, it seems you programmed your website yourself, fully, apart from the forum. On the forum you can ofcourse see everyone so the issue here is the website.
So I assume you have some knowledge about PHP and MySQL cuz you made a login system as well.
Then there is only one thing left. You have to create a table for the online users. What happens is that everytime someone logs in, you put him or her in that table. And if the session timed out (your code checks every 10 min), you should delete that person from the table. Then you display that info of the table in your webpage.
I know it seems awkward when I write it like this (like a story), but that's the explanation for what happens. Next to that, I got a link for you, you could check to see a code that should work. I haven't tested it, but looking at the code it should work.