Assuming that the file and the directory the file lives in (its parent directory ) is owned by a totally separate user, and belongs to a joint group...
Making the parent directory of the file non-writable for the group would make the file non-deletable by the members of that group. By having the file non-deletable by group members, you ensure that the file can't be moved away from its parent directory by those group members.
You can't make the file readable and non-copyable since reading the file is inherently allowing users to copy it.
"Others" would have no permissions on the file or parent directory.
Ideally, you would use some form of system, possibly backed by
git, that would allow people to collaborate on the file in a controlled manner. This would allow for easier access control, remote access, and also for revision control.