Combine user registration/login sessions and user data between a PHP site and Joomla

Background I have a site that is made up of two separate scripts, Joomla and a PHP based freelancer script. I am attempting to make the user data and login credentials common between both sites, so that users do not have to register and login into each script separately. The freelancer script will be the script for registration and creating profiles. Task 1 Combine the user registration/login sessions between the freelancer script and Joomla: Users register in the freelancer script by one of two registration pages. If the user is a client (buyer role), the user registers through [login to view URL], giving them buyer privileges in the script. If the user is an attorney (provider role), they register through [login to view URL], giving them provider privileges in the script. I would like the registration/login information to apply to Joomla as well, so when users register and login through the freelancer script, they will automatically be registered and/or logged into Joomla. This should also apply to logging out as well. If the user registers as a client, they will go into the client user group of Joomla. If the user registers as an attorney, the will go into the attorney user group in Joomla. Task 2 Merge the user data from the freelancer script with Joomla: Ensure that the profile data from the freelancer script applies in Joomla; name, profile pictures, etc.