Linux add Users (user) to user groups (group)

Add a user to a user group and never use it directly:
Usermod-g Groupa
Doing so will allow you to leave the other user groups, just as members of this user group Groupa.
Should be added with the-a option:
Usermod-a-G Groupa User

(fc4:usermod-g GROUPA,GROUPB,GROUPC user)
-A represents append, which is to add itself to the user group Groupa without having to leave another user group.
All the options for the command, and what it means:
Options:
-C,--comment comment new value of the GECOS field
-D,--home home_dir new home directory for the user account
-E,--expiredate expire_date set account expiration DATE to Expire_date
-F,--inactive inactive set password inactive after expiration
To INACTIVE
-G,--gid Group Force use Group as new primary group
-G,--groups groups new list of supplementary groups
-A,--append append the user to the supplemental GROUPS
mentioned by the-g option without removing
Him/Her from other groups
-H,--help display this help message and exit
-L,--login new_login NEW value of the login name
-L,--lock lock the user account
-M,--move-home move contents of the home directory to the new
Location (with only with-d)
-O,--non-unique allow using duplicate (Non-unique) UID
-p,--password password use encrypted password for the new password
-S,--shell shell new login shell for the user account
-U,--uid uid new UID for the user account
-U,--unlock unlock the user account

To view the groups that the user belongs to using the command: $ groups User

or view files: $ cat/etc/group

Linux add Users (user) to user groups (group)