I want to create a new user group.
The members in this user group can create new users and new groups.
I think creating users
You cane manage that through Alfresco Share. In your Alfresco installation directory:
/tomcat/webapps/alfresco/WEB-INF/classes/alfresco/model/permissionDefinitions.xml
you'll find the permissionDefinitions.xml file. Through that file you can manage your custom roles for Alfresco users and groups.
Here, in this WIKI you can find some hints about what you need to change to accomplish that.
Hope it helps. The wiki helps here, if you need some better explanation please let me know.