groupadd: command not found
It seems that groupadd is the correct command: RHEL5 admin. addgroup is a debian specific commnad.
Are you logged in as root? groupadd is in /usr/sbin which, by default, is not in the path. run echo $PATH and see if /usr/sbin is there. Not that running it as non-root is very helpful: you require root access to add a group.