sequential autoresponder upgrade

The requirement is to upgrade an autoresponder php script in order to have the functionality of an online autoresponder service with the ability of giving people sequential autoresponder accounts with username and password. Basically I have an open source sequential autoresponder script and it is a single user (only administrator) system - it cannot handle multiple accounts (users) on the database. This script needs to be modified to have the capability of giving out accounts to an unlimited number of users by the administrator after they have signed up and paid for the service. The database needs to be expanded to accommodate unlimited users on the database, each with its own permissions and logon and a php file needs to be developed for me to manually run the formation of each new user on the database (populating the database with his/hers initial data.) Please no offers of selling me complete scripts. It is a quick job for an experienced coder. Thank you