@marsm 

Posted by: @marsm

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I assume that this go into wpforo's CSS section? 

No, this isn't CSS

The code above belongs in footer.php, because that's where you want to show most users online.

But I'm not sure if you should really make changes to wpForo's theme files. You should only do that if you know what you're doing.
I think the widget is the better alternative for you.