chmod calculator
http://www.javascriptkit.com/script/script2/chmodcal.shtml
accidently remove yourself from all groups? How to recover
http://ccollins.wordpress.com/2007/07/02/restore-default-ubuntu-groups/
The correct syntax when adding yourself to a group is
sudo usermod -a -G groupname username
who is in what group is in the file
/etc/group