Explanation
The Samba service that handles the membership of a file server in an Active Directory domain is winbindd.
Winbindd is a daemon that provides a number of services to the Name Service Switch (NSS) capability of the system, such as resolving user and group information from a Windows NT server and authentication. Winbindd can also be used to join a Samba file server to an Active Directory domain and authenticate domain users to access the file shares12 References:
Chapter 4. Using Samba for Active Directory Integration Red Hat Enterprise Linux 7 - Red Hat Customer Portal Winbind: Use of Domain Accounts - SambaWiki