newgrp: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/newgrp - from package: shadow-utils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/newgrp - from package: login

You may install the required package with command apt-get install {package}

Error in other languages:
newgrp: bevel nie gevind nie
newgrp: comandă negăsită
newgrp: comando não encontrado
newgrp: commande introuvable
newgrp: command not found
newgrp: käsku ei ole
newgrp: không tìm thấy lệnh
newgrp: komanda nerasta
newgrp: Kommando nicht gefunden.
newgrp: kommandot finns inte
newgrp: komut yok
newgrp: nie znaleziono polecenia
newgrp: níor aimsíodh an t-ordú
newgrp: no se encontró la orden
newgrp: no s'ha trobat l'ordre
newgrp: opdracht niet gevonden
newgrp: parancs nem található
newgrp: perintah tidak ditemukan
newgrp: príkaz nenájdený
newgrp: příkaz nenalezen
newgrp: команда не найдена
newgrp: командата не е открита
newgrp: 命令找不到
newgrp: コマンドが見つかりません