1) Login to UBB as the administrator, then go to the Control Panel and enter the Template Editor.

2) Select the ubbthreads.tmpl template, and click on Edit Selected Template:

3) Scroll down until you find the highlighted line:

Then, replace it with:

<td colspan="4" class="tdheader">

Scroll down again, near the end of the template code, until you find this section:

Place your cursor directly before the last </tr> tag and paste the following code:

<td width="25%" class="lighttable" valign="top">
<iframe height="100" frameborder="0" src="chat/info_embedded.php" style="width: 100%; margin: 0; padding: 0; border: 0; overflow: hidden;" scrolling="no" border="0" allowtransparency="true"></iframe>

You can tweak the height parameter of the <iframe> tag to adjust the height of the userlist.

Then, click on the Update button.

4) Now, scroll down to the bottom of your forum's home page. You should see something like this: