The setfacl utility sets Access Control Lists (ACLs) of files and directories.
To install setfacl on Ubuntu/Debian, run
apt install acl
Here is an example, where we create 2 users, one user will be given access to folder inside another users home directory.
useradd -m --shell /bin/bash user1 useradd -m --shell /bin/bash user2 su - user1 mkdir share setfacl -m u:user2:rwx share
in setfacl, -m option is used to modify. u specify user, if you need to set acl for a group, use g instead of u.
See Linux Commands