I've been trying to set this up, but I can't find a solution. It's neccessary, because the machine Ubuntu is on is a Macbook and needs to have access to the files of the Mac user. Is there any way to get this to work?
LightDM – How to Make Users with UID Under 1000 Appear in LightDM
lightdmmacbookusers
Best Answer
Lightdm has the config file
/etc/lightdm/users.conf
which you can edit to set a minimum UID. The default is 500. It also sets hidden users and certain shells that if a user has are hidden. Try editing that file and restarting lightdm.