Custom Joomla development and Integration with PHP website

We currently have a website which was created in Joomla. Extensions included in the site (which are relevant to this project) are Community ACL, Account Expiration Control and Community Builder. We have an external website which was created in PHP. We need to integrate the two so that when a user purchases a subscription via Account Expiration Control they are added to the relating membership in Community ACL (this functionality is already in place) and are then syncronized so their email/ username, password and access level (either entire site, section A or section B) are passed through to the external PHP website. This user can then, once logged into our joomla site, select a tab to go through to this PHP website without having to login again (automatically logs them in to the external website seamlessly). The integration mentioned above can be coded differently as long as the above functionality works (user purchases subscription to access external website and is automatically logged in). The next part of what we require is that this user becomes a "master user" in joomla for this module. They should then have the ability within joomla to add "sub users" to their account. These users will have the ability to also access the external website without having to purchase a subscription or login to the external website. The master user will be able to add sub users that have the same email address domain as them (i.e. can add sub users that also have @[login to view URL]). Users with gerneral domains (i.e. @[login to view URL] or @[login to view URL]) will be required to change their email address to a company one before adding sub users. There needs to be the ability to specify "blacklisted" email domains that will need to change their email address to add sub users. Please feel free to ask questions about functionality if required.