While visiting as a quest in any section of the site other than the main folder, the join.php page thinks that it is in the current sub section that you are currently visitnig, instead of back at the main section.
Example:
www.yourdomain.com/join.php is where the link shoul lie when clicked from anywhere
www.yourdomain.com/forum/join.php is not where the join.php page should go
the same is true for /photos/, /videos/, /blogs/, /anywhere/