I recently purchased a new mini mac to use as a file server and migrated all the files and setting from the old mini (snow leopard). The new mini allows file sharing but not screen sharing unless I remote in with the admin name and password of the administrator of the new mini. I also upgraded the iMacs from snow leopard to yomemite and have the exact same problem. System preferences > screen sharing. I checked the box only these user and added the name from users and group that I want to all screen sharing. But when I exit system preferences, the names disappear. I can add names to file sharing but not to screen sharing.