User Groups are used to assign access privileges to groups of similar users. Once a User Group is defined, you can assign users to it here or in Settings > Users. Defining a group's privileges is accomplished in the Regions and Categories sections of Setup.
Example: You want to create a new group called "Smart Group" and assign a user to it.
How To: Start at Settings > User Groups On the "Manage User Groups" screen, click Add New.

Note the Purchase Override Days field. This populates with the default value from Configuration "Default Admin Product Search Override length in days". You can enter a lesser value to restrict this user from overriding the Purchase End Date by a fewer number of days than default. (Note: the override the user has is determined as the lowest number from all of their user groups and user setting - so if they are in a group with 5 days and have a user setting of 10 days, the user will experience a 5 day override length.)
Note: The pre-seeded groups cannot be modified with regard to name or made Inactive. Any groups you create, however, are modifiable and can be made Inactive by clicking Edit.

Next add a user by clicking Manage Users.
Select the user you wish to assign from the Available column.
Click the right arrow to assign the user to the group.