useradd -m -g initial_group username
-g/--gid: defines the group name or number of the user's initial login group. If specified, the group name must exist; if a group number is provided, it must refer to an already existing group.
If not specified, the behaviour of useradd will depend on the
USERGROUPS_ENAB variable contained in
/etc/login.defs. The default behaviour (
USERGROUPS_ENAB yes) is to create a group with the same name as the username, with GID equal to UID.