You should be running as the user who installed ubuntu, if not, you may not have permissions to sudo
Try the following in a terminal window:
This should list the groups your username is in.
For me, it looks like this:
tredegar adm dialout cdrom floppy audio dip video plugdev lpadmin scanner admin
It should ask for your
password and then let you become root.
Then you can (now you are root) try the command chgrp groupname /path/to/the/file
You'll have to change "groupname" to the name of a group that already exists.
at the terminal to cease being root, before you do something by mistake.
tedegar when I use that command I keep getting a invalid user messages
This really doesn't help me understand what is happening at all.
In future, please copy & paste the commands you gave, and the exact