Users are registered visitor. To be a user of No-CMS, one need to register. The registration can be done via registration page or by using third-party authentication (if activated). You can manage users by accessing CMS Management | User Management.
A user can be a member of several user group, and a user group can contains many user. Group represent user's role. Since a user can has several role, therefore he/she can also be member of several group. User group is important for page with authorized authorization type. You can manage group by accessing CMS Management | Group Management.
A group can also asociated with navigation and privilege
You can manage user and user-group relation by using both, group management or user management.
PS : if a user is a member of Super Admin user-group, he/she will always be authorized to view any page and has any privilege. Be sure whenever you want to assign a user into Super Admin user group.