UBB.threads PHP Forum Software Community Forums UBB.threads Version 7/8 Support Placing banners conditionally

https://www.ubbdev.com/forums/ubbthreads.php/topics/316341/7_2_2_Random_Adds.html#Post316341

And just find the template you want to to add an add. I guess in your case postlist.tpl

In the code you could choose to display it only for certain groups (or remove it for certain groups).