Select the "User Management" tab on the left side of the screen, and ensure "Share users/groups" is selected at the top of the pane. Notice the note that "only one authorization protocol can be enabled in the same time. Changing the protocol will result in the loss of saved data".

Active Directory (AD) server authentication allows you to connect to an existing AD server. You can also set the server authentication settings from this page by filling out the fields inder "AD server authentication settings" heading. 

 Alternatively, you can configure the LDAP domain name and password by filling out the fields shown below.

After applying these settings, you will be prompted to allow the SMB to restart. Select "Yes" and wait until the SMB is restarted.