Hi Ikrananka,
if you install D-Fend Reloaded in "Program directory mode" all user will share the same settings and installed games. But (at least under Vista or under XP in non-admin mode) you will have to take care all users have the permission to write to the D-Fend Reloaded program directory.
If you want to use the normal "User profile mode", you can still store your games in any directory (including something like C:\Users\Public\Games). By setting the "games directory" in the setup dialog to this common directory, it will be offered in the profile editor as the default place for games. But the profiles will be individual per user in this mode, so you will need to copy them to each user profile. (In this mode you won't have to worry about write permissions, because there will be no write operations in the program directory.)
You can also install DFR in portable mode into a common directory (like C:\Users\Public\D-Fend Reloaded). Using a portable installation in this directory means all user will share everything and you do not need to care for write permissions - but you will not get a start menu item from the installer, so you will need to create a link in the start menu on your own.