I am guessing you want to send guests to the registration page after viewing a thread....
I am sure someone has a better way but this should work...
Place this at the top of your showthread template
Make sure to enter the correct path to your register.php file and you can change the amount of time before the redirect by changing the content value from 10 seconds to whatever you want.
HTML Code:
<if condition="is_member_of($bbuserinfo, 1)">
<meta http-equiv="refresh" content="10;url=http://www.yoursite.com/register.php">
</if>