Design is nice and clean, text is tiny though, for it to be easily readable I had to whack Opera's zoom up to 150%; also, when you hover over someone's name the layout expands which isn't that easy on the eye.
My real critism though comes with the displaying of the emails, spam bots would have a field day on this site harvesting all the email addresses, maybe implement some sort of image of the email generated via gdlib (remember to use obscure file names though), or even have it so they click on the related messaging client icon and are they are then forced to complete a captcha before they're allowed to actually view the email address.
Additionally, you really need to lock down the form, I registered with the
email 'rthrt' and password of '', also, try and turn off 'gpc_magic_quotes' as it's a bad habbit to become reliant on them, you should be writing your code securely and not relying on them. They also create a myriad of problems, for example you put a quote in your profile and it comes out as "\'", you can of course strip them, but the best plan of action is to disable them COMPLETELY and code with security in mind.
Form validation is a must, php5 comes with a great library called 'filter', check it out
http://www.php.net/filterEmail validation is as simple as:
CODE
$validEmail = filter_var($_POST['email'], FILTER_VALIDATE_EMAIL);
if($validEmail !== true)
// implement error handling
I appreciate the idea and wish you the best of luck.