Creating and managing groups allows you to fine tune the rights you associate to a page or a set of page.
For now, the only rights that can be set is the write rights. I hope I'll be able to add comment/move/view/attach/'plugin defined' rights later.
Anyway, this is here you will create groups and assign users to a group.
Simply click on the group's name to see the composition of the group.
Use the ">>" and "<<" button to add new members of the group or to remove some.