add_members: 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/lib/mailman/bin/add_members - from package: mailman

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

On Debian possible default full paths are:

/usr/lib/mailman/bin/add_members - from package: mailman
/usr/sbin/add_members - from package: mailman

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

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