dropdown-groups
Think of User Groups as Roles. What Role will these people have in the website?  Will they be writing Articles, will they need to have control over everything on the site, will they just need to see certain content on the Front End that is restricted from the public?

Joomla comes with a set of default User Groups, but you may create additional groups.

The default User Groups are Public, Guest, Manager, Administrator, Registered, Author, Editor, Publisher and Super Users.  User Groups are used to set Access Levels and Permissions. Access Levels modify what the User Groups can SEE.  Permissions modify what the User Groups can DO. Access Levels are set under the User dropdown menu, Permissions are initially set under Global Configuration in the System dropdown section, but there are many other places to finely tune your Permissions settings.

Inheritance

Also affecting the User Groups is inheritance aka child-parent relationship.  When you set a Permission for a parent group, that permission is automatically inherited by all of the children of that group.

Group TitleInheritance
 Child ofParent of
Public    All Groups
|-Guest  Public  
|-Manager  Public  Administrator
|-|- Administrator  Public, Manager  
|- Registered  Public  Author, Editor, Publisher
|-|-Author  Public, Registered  Editor, Publisher
|-|-|- Editor  Public, Registered, Author  Publisher
|-|-|-|- Publisher  Public, Registered, Author, Editor  
|- Super Users  Public  

 

The question now is, how do I set the Access Level for the User Group? 

Go into Users -> Access Levels.  Select an Access Level to which you want your User Group Associated.   When you click on the Access Level you will reach a screen with a list of all of the available User Groups.  Clicl on all of the User Groups that you want to have that level of access.  

 

access-registeredEach User Group is associated with Access Levels. Within each Article, Category, Module and Menu item, there is a pulldown where you can set Access Levels. 

Read More about Access Levels...

Each User Group is associated with Permissions. The Permission tab of the Global Configuration menu allows you to set global permissions for User Groups.

Read More about Permissions...

See the User Group Tutorial...