Strategy:

1) create a shared group
2) create a shared directory: /home/shared
3) add all users to this group
4) fix permissions from shared folder and future files inside

$ getent group | grep -i shared          # first check if group exists

$ sudo groupadd shared

$ sudo mkdir /home/shared
$ sudo chmod 770 /home/shared/
$ sudo chmod +s /home/shared

$ sudo usermod -a -G shared user1
$ sudo usermod -a -G shared user2

$ sudo chgrp shared shared              # change group from shared folder

Tips

Files inside 'shared' should be 660.

$ chmod 660 

Directories inside 'shared' should be 2770 (770 with SetGID)

$ chmod -R 2770