It wasn't obvious from the documentation whether with sssd-libwbclient (only, ie without sssd-winbind-idmap installed and configured in smb.conf, since sssd-winbind-idmap is not available in most versions of RHEL7 as it was only recently added),
Samba's uid_to_sid(function) can always do the lookup uid_to_sid to AD if using winbind but it wasn't clear whether this would work with sssd-libwbclient (only) installed and what additional Samba configuration is needed for that.
Without this the owner of a file (viewed from the WIndows client) from Explorer GUI looks like
"Unix user\10000" rather than "user@domain" (as it would for Windows to Windows, or if Winbind were running on the Samba server joined to AD)