That wouldn't do what you want. Only root has permission to reboot/halt the machine (for obvious reasons). I suppose that if you really wanted to do that you could make halt setuid root and make it only executable by owner and group, but I really wouldn't recommend that (I avoid making anything setuid for security reasons). If you use GDM or KDM for your X logins, those can be configured to allow reboot/shutdown from the console without a root login.