sudo usermod -a -G linuxadmins myuser
usermod: group 'linuxadmins' does not exist

[my_sit@myhost /]$ getent group
linuxadmins:x:1000:some_ids
root:x:0:my_sit,other_sit
bin:x:1:bin,daemon
daemon:x:2:bin,daemon

[my_sit@myhost /]$ cat /etc/group
root:x:0:my_sit,other_sit
bin:x:1:bin,daemon
daemon:x:2:bin,daemon