3.4.2. Attaching New Users to Groups

The useradd command creates a User Private Group (UPG, a group assigned to a user account to which that user exclusively belongs) whenever a new user is added to the system and names the group after the user. For example, when the account robert is created, an UPG named robert is created at the same time, the only member of which is the user robert.
If you do not want to create a User Private Group for a user for whatever reason, execute the useradd command with the following option:
useradd -N username
Instead of automatically creating UPG or not creating it at all, you can specify the user's group membership with -g and -G options. While the -g option specifies the primary group membership, -G refers to supplementary groups into which the user is also included. The group names you specify must already exist on the system.

Example 3.7. Adding a User to a Group

~]# useradd -g "friends" -G "family,schoolmates" emily
The useradd -g "friends" -G "family,schoolmates" emily command creates the user emily but emily's primary group is set to friends as specified by the -g option. emily is also a group member of the supplementary groups family and schoolmates.
Provided the user already exists and you want to add them to certain supplementary group(s), use the usermod command with the -G option and a list of groups divided by commas, no spaces:
usermod -G group_1,group_2,group_3