I've created a NextGen project where I would like to restrict a specific User to only see this project. Every other project that User should not be able to view/access it. How is it possible? With a classical project, you can have user groups and limit them. How does it work on NextGen?
As you mentioned, you can control higher-level access to projects with user groups, limiting the visibility this user has into classic projects. In next-gen, user permissions are controlled with the project's access level configuration.
You can change the access level by going to the next-gen project 'Settings > Access'. At the top of the page you will find a 'Change project access' button which will open a dialog offering you three options: