[Chris M]

Open root/forumdisplay.php
Find:

} else {
  // there are no threads or announcements to show
  eval("\$forumdisplay[threadslist] = \"".gettemplate('forumdisplay_nothreads')."\";");
}

Replace with:

} elseif ($bbuserinfo[usergroupid]=="1") {
  // display login or register message to unregistered
  eval("\$forumdisplay[threadslist] = \"".gettemplate('forumdisplay_loggedout')."\";");
} else {
  // there are no threads or announcements to show
  eval("\$forumdisplay[threadslist] = \"".gettemplate('forumdisplay_nothreads')."\";");
}

Create a new template called: forumdisplay_loggedout

To see the contents of this forum, you must first login or <a href="http://$bburl/register.php?s=$session[sessionhash]&action=signup">register</a>

Untested but should work

Satan