We currently have fedora 28 running on our family computer with the Cinnamon desktop environment. Unfortunately, multiple family members have different preferences in DE. Is it possible to have Cinnamon for one user and KDE for another user? How do you set this up?
Yes, easily. There is a little gear icon by the sign-in button on the login screen. Clicking that icon will let you select any of the installed desktop environments. The choice is remembered per-user.
Of course, you do need alternate desktop environments installed for this to work. Generally, you can install them from the command line using
dnf groupinstall. Get the list of various possible groups with
dnf grouplist. (Note that this doesn't necessarily work in reverse; removing a group you decide you don't want may remove things needed other groups.)