Create a folder such as /home/Common
 Change permissions. This depends on what you mean by "shared":
sudo mkdir /home/Common
This will allow all users to add to and delete any file but only be able to read and not write to someone else's files:
This will do the same thing as above but only the owner of the file can delete the file ( the "1" is a "sticky bit" ):
sudo chmod 0777 /home/Common
If you want the ability to actually write to another users files then things get more involved:
sudo chmod 1777 /home/Common
** Change the group of the folder to say for example, plugdev:
** If your users are not members of the plugdev group they will need to be added:
sudo chown :plugdev /home/Common
** Then you need to change the default umask so that files are added as 664 and not 644:
sudo gpasswd -a morbius plugdev
NOTE: The default umask is already at 002 if you are running 12.04 so this step is not required.
Edit /etc/profile as root:
Find the line at the bottom that references umask and change it to:
gksu gedit /etc/profile
** Then change permissions of the shared folder:
sudo chmod 2775 /home/Common
"2775" - All plugdev users will be able to add and delete all files and will have read/write access to all new files. The "2" is a setgid bit.
sudo chmod 3775 /home/Common
"3775" - All plugdev users will be able to add to but delete only their own files and have read/write acces to all new files. The "3" is a "sticky" (1) + "setgid" (2) =3.