A group is a collection of users which can be managed together as if they were one user. Groups can be granted system-level permissions or roles on weblogs. Groups can be enabled and disabled.
Your ability to add, edit, or delete groups will be different depending on whether you are managing Movable Type Enterprise Natively or Externally. Please refer to the relevant sections Chapter 2 of the Users and Administrators Guide, entitled "Configuring Users and Groups" for more detailed information.
You can download the Users and Administrators Guide here.