Mambo Open Source/User management
Mambo users are the same as logins, and can be used to many great things. Here is an explanation of the management of users, not what you can use the user system for.
Basics of user management 
There is not much that you can do with mambos integrated user management. You can add or edit users, force logout of users, apply global checkins and mass mail user groups. That is pretty much it, here is an explanation of each.
All these actions are done in the backend.
Add, edit or remove a user 
Go to "site > user manager" in the drop-down menu. There, you will get a list of all the users in the database. In the toolbar, press new, delete or edit to do the corresponding action.
Note: if you delete a user, you cannot revert it unless you have a backup of the xxx_users table in the database.
Global checkin 
If items are erroneously checked out, you can perform a global checkin with the "system > global checkin" function in the drop down menu. Now, all items will be checked out.
Extended user management 
A text about the possibilities to extend user management will come here (components, hacks etc).