I have a WordPress website for registered users only.
Right now only users created manually through the WP admin interface can get access.
We are currently registering all our users in a newsletter system, stored with: name, e-mail, company and password.
So we need an integration between the WordPress site and the Newsletter system, so we can give all users registered in both system access to the main WP website.
There is a well documented API for the newsletter system.
Attached is screendumps of the Newsletter system called: A P S I S
Also the API documentation is attached.
Most important is that our admins only have to keep one userlist updated and it has to be the A P S I S list.