Access Control Lists
An extension to standard UNIX-style file system permissions.
Sadly the built in UNIX way of doing permissions, with it's notions of users, groups, and "others" is in practice quite difficult to use to make "shared folders". ACL (for access control lists) is an extension to these mechanisms that gives more flexibility. The setfacl command is the thing that sets the options.
sudo apt install acl
So, the command to Allow anyone in the users group to write files to path/to/some/folder
setfacl -Rm g:users:rwX /path/to/some/folder setfacl -d -Rm g:users:rwX /path/to/some/folder