User groups allow easier access control maintenance, as users with similar permissions can be administered together once rather than separately several times. The special group Everyone always exists and cannot be edited; it always contains all users as a convenience.
User groups may contain other groups as well as users, allowing complex hierarchies to be created if needed. Permissions for a user are affected by all groups a user is directly or indirectly a member of. For example, if user Amy is in group Programmers, and group Programmers is in group IT, then Amy is also indirectly a member of IT, and her permissions are affected by those granted to not only herself and Programmers but IT as well.