groupmems: 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/sbin/groupmems - from package: shadow-utils

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

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