Thank you very much that is very kind of you. :)
No problem!
Well, I found two ways of doing this (or rather one easy way and one way that looked really convoluted and would take me hours to completely track down).
So here's the easy way - its not as pretty but it should do the trick I think:
Remove the following:
include/client/open.inc.php
<input type="text" name="email" size="25" value="<?=$info?>">
and insert
<? if(!($errors)){?>
<input type="text" name="email" size="25" value="@yourdomain.ext">
<?}else{?>
<input type="text" name="email" size="25" value="<?=$info?>">
<?}?>
This way all they have to do is type in their username before the @ symbol. Hopefully they don't get confused in the process ;)