Depends on what you’re looking for? Common logins? A way of mass applying configurations and policies or to multiple computers? Way of doing centralized shared file stores?
There’s no true 1:1 in Linux, but there are multiple applications that can cover all of the functions. As one person said, LDAP, but that’s a protocol that can be served via multiple applications. Samba is one that offers an AD like system that would probably cover SoHo type needs. Things like openldap, 389 server and other can do pure directory/authentication but may not meet everything.