lege zuerst mit adduser oder useradd (adduser ist etwas einfacher) die benzuer an, danach musst du den usern mit smbpasswd passwörter vergeben.
*edit*
schau dir wirklich mal die man page von smbpasswd an
dort stehen interesannte infos drin:
-a This option specifies that the username following
should be added to the local smbpasswd file, with the
new password typed (type <Enter> for the old password).
This option is ignored if the username following
already exists in the smbpasswd file and it is treated
like a regular change password command.
Note that the
default passdb backends require the user to already
exist in the system password file (usually
/etc/passwd), else the request to add the user will
fail.