Like most things with Linux simple questions often result in complex answers - and in this case it all depends on what you mean by "shared"
Let's say you create a folder: /home/Shared
The following configurations will allow multiple users to add to and delete from the shared folder.
This will allow all users to add to the folder:
This will restrict it to only members of the plugdev group:
sudo chmod 0777 /home/Shared
The last command adds morbius to the plugdev group.
sudo chmod 0770 /home/Shared
sudo chown :plugdev
sudo gpasswd -a morbius plugdev
If you want users to be able to do the above and also to edit the files contained within that shared folder things get a bit more complicated.
This will allow all users of the plugdev group add/delete access to the folder and edit access to any new files created in or copied to the folder:
Note: What it will not do is affect any files moved to the shared folder. For that you need bindfs.
sudo chmod 2770 /home/Shared
sudo chown :plugdev /home/Shared
Another Note: If you are using Ubuntu13.10 you will unfortunately have to make one correction to the system:
EDIT: I had to edit this post since on a fresh install of 13.10 even this didn't work correctly. It looks like in 13.10 you will need to use bindfs if you want to edit the contents of a shared directory.
Last Note: You can also add a "sticky bit" to these that will prevent one user from deleting a file owned by another which I can explain if desired.