What is an additional task which would have to be carried out by a multiuser operating system?

Answer 1

Between a single user and a multi user operating system?

Generally the kernel of the multiuser operating system has to take care that one user can't interfere with the other, much like a multitasking operating system has to protect processes from each other.

Also much like a single user multitasking system, any normal user absolutely must not be able to interfere with the system. This is generally why anything outside of ~ (/home/username) is read/execute-only at most for a user, except in cases where a user needs to be able to write out data, too, such as /tmp.

Probably the best example is how security and access control is implemented. It's all highly user-oriented: A user will have rights over files they control and files they are granted rights to only. Keeping in mind there's different types of access and most users' write permissions are restricted to a user's own personal space.

This access control varies. And in some cases the same multi-user system can be configured by its administrator to use many different kinds of access control. For example, Linux offers SELinux, Tomoyo, Apparmor, PaX, GrSec, or its default and plain vanilla access control lists wherein the system tracks user and group ownership of a file and determines what rights the user owner has, what rights the group owner has, and what rights non-owners have.