97294434_Linux_useradd_Command_Examples__Option_Description
[Short Option] -g [Long Option] –gid GROUP [Option Description] 
The group name or number of the users initial login group. The group name must exist. A group number must refer to an already existing group. If not specified, the bahavior of useradd will depend on the USERGROUPS_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.
5 UNIX / Linux useradd Command Examples
Option Description