Groups
Operating systems such as Linux, and Mac OS, (and Windows 10???) have a notion of groups to manage collaboration of the users of the system.
Check the groups of a user
groups
groups username
Add an Existing User Account to a Group
usermod -a -G group username
Create a new group
sudo groupadd mynewgroup
Change the group of a file/folder
chgrp group fileorfolder
To change the group of a folder and all its contents, use the -R (recursive) option:
chgrp -R group folder
For even more social/sharable folders, see ACL.