i.group: 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/lib64/grass-6.4.4/bin/i.group - from package: grass

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

On Debian possible default full paths are:

/usr/lib/grass64/bin/i.group - from package: grass-core

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

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