A página de manual diz algo diferente. Citando o OpenSUSE 13:
By default, a group will also be created for the new user (see
-g
,-N
,-U
, andUSERGROUPS_ENAB
).
e
-g
,--gid
GROUP
The group name or number of the user's initial login group. The group name must exist. A group number must refer to an already existing group.If not specified, the behavior of
useradd
will depend on theUSERGROUPS_ENAB
variable in/etc/login.defs
. If this variable is set to yes (or-U
/--user-group
is specified on the command line), a group will be created for the user, with the same name as her loginname. If the variable is set to no (or-N
/--no-user-group
is specified on the command line),useradd
will set the primary group of the new user to the value specified by the GROUP variable in/etc/default/useradd
, or 100 by default.
Na minha VM do OpenSUSE 13, a configuração em /etc/login.defs
é (o padrão) não .
No OpenSUSE 12, esse recurso não existia.