is the default group "users" specific?

A long time ago, common practice was to create user accounts and have them share a group, often based on their role. For example, in a university setting, students would be members of a students group, and staff members of a staff group. Users would configure their umask depending on the level of privacy they wanted; in trusting (or more cynically, naïve) environments, a 022 umask was common (preventing group members from being allowed to write to files they don’t own). users is intended for this kind of setup.

This creates problems on two levels. The first is that users end up paying more attention to “other” permissions than group permissions when thinking about privacy. The second, ironically, is that this type of setup creates problems when working in directories which are supposed to be shared!

To understand the second problem, consider a typical (back then) shared web server content directory. You might have a group, say www, containing users allowed to work on the web server’s content (and the web server daemon user itself). Directories would be created sgid, so that new files belong to the www group. But the default 022 umask would prevent group members from editing each others’ files, even with www group ownership.

Switching to “user-private groups” solved both problems, and that’s the common model today. For historical reasons however, it’s still helpful to preserve the common groups that were used in the past; this includes users. The users group is not generally used nowadays, no longer has a specific role, and doesn’t have default privileges attached.

See also Where to find purpose of each system default Groups and Users in Linux and The How and Why of User Private Groups in Unix.