how do i restrict access to a sharepoint 2016 web application to an active directory group

Hi @Greg Booth ，

Since the AD group cannot be directly sync to the SharePoint server, so users in the AD group can only be manually added to Policy for web application in SharePoint 2016.

To manage permissions for a web application

1. Start SharePoint 2016 Central Administration.
2. On the SharePoint Central Administration website, in the Application Management section, click Manage web applications.
3. In the web applications list, click the web application for which you want to manage permissions.
4. In the Security group of the ribbon, click User Permissions.
5. In the User Permissions for Web Application dialog, select the check boxes next to the permissions that you want to enable, and clear the check boxes next to those permissions that you want to disable.
6. You can select all permissions by selecting the Select All check box. You can clear all permissions by clearing the Select All check box.
7. Click Save.

Reference:https://learn.microsoft.com/en-us/sharepoint/administration/manage-permissions-for-a-web-application

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