How is the user guest and/or the group guest supposed to work on Linux/Unix system? How about the "Guest Session" when using Ubuntu with the new display-manager?

Can I create some new users and give the the 'guest'-group as default group, and thus automatically give them less privileges (in a way similar to the 'Guests'-group in Windows)? How?

Should the group (in Linux/Unix) be called 'guest' or 'guests'?
Should the user 'guest' be member of the 'guest(s)' group, or some other group (nobody, nogroup, users)?
If a system have both a user called 'guest' and a group called 'guest', should the UID and GID preferably be a pair (ie. same number, eg. both set 120)?