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,--gidGROUP
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
useraddwill depend on theUSERGROUPS_ENABvariable in/etc/login.defs. If this variable is set to yes (or-U/--user-groupis 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-groupis specified on the command line),useraddwill 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.