In PHP, you would:
make a form that sends the username and password... via POST, of course.
<?php
exec("adduser -u $HTTP_POST_VARS[user] -p $HTTP_POST_VARS[password]", $results);
?>
of course, it would be a good idea to use escapeshellcmd() (I forget the exact spelling, sorry - it's early and I haven't had any coffee), because a naughty user might semi-colon off your adduser command
Of course, the options for adduser vary per system... play with them and figure out what works!