EXEC master.dbo.sp_addlinkedserver
    @server = N'ADSI',
    @srvproduct=N'Active Directory Service Interfaces',
    @provider=N'ADSDSOObject',
    @datasrc=N'MySub.MyDomain.com'

EXEC master.dbo.sp_addlinkedsrvlogin
    @rmtsrvname=N'ADSI',
    @useself=N'False',
    @locallogin=NULL,
    @rmtuser=N'MyUser',
    @rmtpassword=N'MyPassword'

SELECT *
FROM OpenQuery
(
    ADSI,
'SELECT name
FROM ''LDAP://MySub.MyDomain.com/DC=MyDomain,DC=com''
WHERE objectCategory = ''user''
AND samaccountname = ''johndoe''
AND memberOf=''cn=IT GROUP,OU=Groups,DC=MyDomain,DC=com'' '
)