system level adduser

We're creating a webmail application using U-Wash IMAP. The user fills out a signup form to get an account. We want to add the user in real time, giving them access to their account immediately. The best idea right now is to loop through the /etc/passwd file, looking for the existence of a system username. If the selected name is not on the list, we'd like to add it (probably using 'system()'). Would anyone care to comment on the security and procedure of adding system users from a php script? - John Hogan