I want to retrieve users/ a user from an AD-group in SharePoint 2016 and show them on the permissions site under site settings. Add a custom button for this first under the permissions menu for example New, and then display all users from within that AD-group. Is this possible with client code (no back-end code if possible) and how do i proceed/start this process? The only one who should have this access to retrieve the users is the site admin. Anyone that has an idea how to do this? Thank you!
that's not possible, you cannot customize site settings pages, you'll have to write your own sandbox or farm solution to execute C# code to extract the users in the AD groups and place your webpart in a new page.