Probably /usr/sbin is not in your PATH and neither is . (the current working directory). Are you getting a root shell by doing su? If so, you're not getting root's environment, which has the sbin directories in its PATH. Use "su -" (note the trailing dash) instead. You can easily modify your PATH -- search the forums for details, how to do it has been discussed many times.
For now, see if "/usr/sbin/groupadd" (no quotes and note the leading slash) works for you.