Ubuntu server 18.04 user permissions

Question

I've set up an Ubuntu Server 18.04 where I have a Dropbox app running over user0. This app is set up to make a directory backup. I have two directories: /home/user/dropbox/user1 and /home/user/dropbox/user2. Both folders ownerships are set to user0. I created two more users on the server, user1 and user2. How can I give each user read and write permissions to these folders? This folders will be accessible from a Samba share where user1 will work with /home/user/dropbox/user1 and user2 will work with /home/user/dropbox/user2. user1 should not be able to see user2 files. Thanks!