OK, the page you're talking about is called FORUM HOME, so go to Styles & Templates -> Style Manager -> Edit Templates for your style(s)... Now go to the Forum Home template... On this page find:
Code:
<div>$activeusers</div>
That is the code that displays the online users, so replace it with the following:
Code:
<div><a href="http://www.YOURDOMAIN.COM/forums/member.php?u=USERID">Phantom User</a>, $activeusers</div>
Replace YOURDOMAIN.COM obviously with your domain name.
Replace USERID with the user id number of whatever username you are using.
Replace Phantom User with the username
This is for 3.8, may be similar for other versions.