Understanding Multi-Person Directory and File Permissions

It may depend on the reason for separating users, but when sharing files between different UNIX users, the following methods may be considered:

(a). Create a UNIX group for sharing and join each user to that group
(b)Specify permissions per user using the file system ACL feature (POSIX ACL)

(a) is a group-by-group permission designation, so I think it is difficult to specify an exception only for a specific user.

(b) can specify permissions in detail on a per-user basis, but may require remounting of partitions in some environments.

Also, if you don't mind giving the same access across shared users,
"Create a single UNIX user for sharing and use public key authentication to register and manage each user's public key with a UNIX user for sharing."